Thomas Leonard's blog

Replacing Python: Second Round

In the first post, I took a brief look at the programming languages ATS, C#, Go, Haskell, OCaml, Python and Rust to try to decide which would be the best language in which to write 0install (which is currently implemented in Python). Now it’s time to eliminate a few candidates and look in more detail at the others.

Last time, I converted 4 lines of Python code from 0install into each language. This time I’m converting 576 lines, so this should give a more accurate picture of how each performs for a real-world task.

As before, note that I’m a beginner in these languages. Please post corrections or suggestions of better ways of doing things in the comments. Thanks.

(this post also appeared on reddit and on Hacker News, where there are more comments)

Table of Contents

Conclusions from last time

Based on the initial evaluation and feedback (thanks everyone!), I’m not going to look any further at these:

ATS
I’m glad I included ATS. Its excellent performance and tiny binary put the other languages into perspective. Still, it’s too hard to use, and makes it too difficult to separate safe code from unsafe code.
C#
Although C# is widely used, has an excellent cross-platform bytecode format and many libraries available, it is too large and too slow to start for 0install (to the people who suggested profiling: when even “Hello World” is too slow, there isn’t much you can do).
Go

Although many Go users complained that Go’s score was unfairly low, they didn’t seem to disagree that it was the worst of the candidates for our requirements, only about by how much it was the worst. To summarise the discussion briefly:

  • Go is good because errors are handled where they occur. Maybe, but ATS, Haskell, OCaml and Rust do that too, and they help you get it right.

  • You can write pretty reliable code in Go. No doubt, since you can do it in C too. But maybe I can write even better code with less effort using the other languages?

  • It’s OK to ignore errors silently in some places, because handling all errors would clutter the code up too much. This seems like a trade-off the other languages don’t require me to make.

Rust
Rust has excellent safety and a familiar imperative syntax. It also has excellent support for shared libraries, which I didn’t understand when I wrote the previous post (although I don’t feel too bad about this as it seems that almost no-one else in the Rust community understood it either). Speed is OK but not great, though likely to improve. Rust’s main weakness is its immaturity. The language is likely to change in incompatible ways in the near future and there are few libraries available (for example, there is no XML library for Rust). It will be a few years before this is usable in production (and the developers make no secret of this).

So, here are the remaining candidates and a summary of the conclusions from last time:

Haskell (7.6.3)
Haskell is fast, but it has problems with shared libraries: libraries are only compatible when compiled by the exact same version of the compiler. Its pure functional style may make it very difficult to convert the existing code to Haskell. Diagnostics (e.g. getting stack-traces) may be a problem too.
OCaml (4.00.1)
OCaml is also fast and had good safety, but its support for shared libraries seems limited. Getting good diagnostics from production code may be tricky, as enabling stack-traces has a performance cost (OCaml code assumes exceptions are cheap).
Python (2.7.5 and 3.3.2)
Python is much slower than the other candidates, but it does have the advantages of an excellent standard library, easy distribution (no need to make platform-specific binaries), being the language we’re currently using, and being very well known. But it has no static type checking, which means a lot of work writing unit-tests for even trivial code (e.g. testing __repr__ methods and logging in obscure error paths to make sure it won’t crash due to a typo).

Several people said that D has improved a lot in the last few years. I didn’t have time to look at it carefully again, though. It’s a nice language, but probably too low-level/unsafe for us. For example, it’s trivial to make it segfault by dereferencing a null pointer.

Test case

0install collects information about all available versions of a program and its libraries from around the web. Then it runs a solver to determine the best valid combination and writes the results to an XML selections document. For an app, foo, the current selections can be found in ~/.config/0install.net/apps/foo/selections.xml.

When it’s time to run an application, we read this XML, set up a suitable environment for the process and then exec it. As before, this should happen as quickly as possible.

The test program can be given either the name of an app (e.g. “foo”) or the path to a selections document.

This task involves:

  • Using the XDG basedir spec to find the current selections and the cached software.
  • Parsing XML into a suitable data structure.
  • Manipulating pathnames, files, directories and symlinks.
  • Updating environment variables based on the requested bindings.
  • String manipulation.
  • Creating launcher executables (as in the previous test).

We can’t go over every line in this post, so I’ll just highlight the most interesting bits. The full version in each language is in this GitHub repository. If you get bored, there’s a summary at the end.

Syntax

Python

I guess most people are familiar with Python. It’s clear, simple and straight-forward. It uses indentation to see when a block ends, which means that the structure of a program is exactly what it looks like. Here’s a sample of the Python main module:

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config = get_default_config()
args = sys.argv[1:]
if not args:
	raise Exception("Syntax: runsels.py [APP | SELECTIONS] [ARGS]")

app_or_sels = args[0]
app_args = args[1:]

app_mgr = apps.AppMgr(config)

app_path = app_mgr.lookup_app(app_or_sels, missing_ok = True)
sels_path = join(app_path, "selections.xml") if app_path is not None else app_or_sels

with open(sels_path, 'rb') as stream:
	sels = qdom.parse(stream)

runner = run.Runner(config)
runner.execute_selections(sels, app_args)

Note: The real Python code uses the “optparse” module to handle option parsing and generate help text. However, because the Haskell/OCaml code developed here will be used as a front-end to the real Python, we don’t want any special handling. For example, if invoked with --help they should fall back to the Python version instead of handling it themselves. So proper option parsing isn’t part of this task.

OCaml

OCaml syntax is very compact, but somewhat prone to confusing syntax errors. Often an error reported at a particular line means you used “;” rather than “;;” 20 or 30 lines earlier. Here’s the OCaml main module:

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let () =
  let config = Config.get_default_config () in
  match List.tl (Array.to_list Sys.argv) with
  | [] -> failwith "usage: runsels selections.xml arg..."
  | (app_or_sels :: args) ->
      let sels_path = match Apps.lookup_app app_or_sels config with
      | None -> app_or_sels
      | Some app_path -> app_path +/ "selections.xml" in
      let doc = Qdom.parse_file sels_path in
      let selections = Selections.make doc in
      Run.execute_selections selections args config
;;

Some useful things to know when reading the examples:

  • let a = b in ... assigns a variable (well, constant), like a = b; ... in Python.
  • let foo a b = ... defines a function, like def foo(a, b): ... in Python.
  • foo (a + 1) b means to call foo with two arguments, like foo(a + 1, b) in Python.
  • foo a creates a partially applied function, like functools.partial(foo, a) in Python.
  • () means no data (you can think of it as an empty tuple).
  • Module names start with a capital letter (e.g. Run.execute_selections).

We assign the result of Run.execute_selections to () just to check that it didn’t return anything (so if it did, we’d get a type error rather than just ignoring it).

The syntax tends to emphasise the body of functions while minimising the signature, which I’m not convinced is a good thing. Consider:

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let get_source b =
  let get name = ZI.get_attribute_opt name b in
  match (get "insert", get "value") with
  | (None, None) -> failwith "Missing 'insert' or 'value'"
  | (Some i, None) -> InsertPath i
  | (None, Some v) -> Value v
  | (Some i, Some v) -> failwith "Can't use 'insert' and 'value' together"
;;

What is the argument b here? What is the return type? These things are inferred by the compiler. b is an element because ZI.get_attribute_opt takes an element as its last argument. The return type is env_source, because InsertPath and Value are constructors for env_source objects.

You can include the types, but generally you don’t and that makes it hard to know the type of a function just by looking at its definition in the source code.

Update: ygrek points out that most text editors can tell you the type of an OCaml identifier, e.g. see these instructions for Vim.

It also tends to bulk out the code: e.g. Map.find key map rather than the more object-oriented map.find key. OCaml does support objects if you want them, but the normal style seems to be to avoid them. On the other hand, it does make it very easy to see which bit of code is being called, which isn’t so obvious in an object-oriented style.

One thing to watch out for in OCaml is that, unlike Python and Haskell, it doesn’t look at your indentation. Consider the following code (this is a simplified version of a mistake I actually made):

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let () =
  print_endline "Start";

  match 1 with
  | 1 -> print_endline "one"
  | _ -> print_endline "many";

  print_endline "End"
;;

This code never prints “End”, because it treats that as part of the “many” branch, even with all warnings enabled. It would be nice if it would issue a warning if a single block (e.g. the last match case) contains lines with different levels of indentation.

Haskell

Like Python, Haskell uses whitespace for indentation and generally avoids semi-colons and braces. Here’s the main function in Haskell:

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main :: IO ()
main = do progPath <- getFullProgName
	  absProgPath <- absolute_path progPath
	  conf <- Config.getDefaultConfig (dropFileName absProgPath)
	  argv <- getArgs
	  case argv of
	       [] -> error "Syntax: runsels [APP | SELECTIONS] [ARGS]"
	       (appOrSels:args) -> do
			app <- lookupApp appOrSels conf
			let selsPath = case app of
				Nothing -> appOrSels
				Just path -> path </> "selections.xml"
			sels <- loadSelections selsPath
			executeSelections sels args conf

Some things you should know to help you read the examples:

  • :: means “has type”. Functions typically declare their type at the top.
  • myfunc :: String -> Int -> Bool means that myFunc takes a String argument and then an Int argument and returns a Bool.
  • do notation means that the expressions inside are connected together in some type-specific way (this is quite confusing). Since main has the type IO (), these statements are connected by the “IO monad”, which does them one at a time, interacting with the system for each one. If the do block had a different type, it might do something completely different.
  • a $ b $ c d means a (b (c d)) (without them it means ((a b) c ) d). It helps to avoid having lots of close brackets.

The “run” module

Once we’ve loaded the selections XML document, the basic steps to execute the program are:

  1. For each selection, get the path to its directory (for packages not provided by the distribution).
  2. Scan the XML and collect all the bindings (things we need to set up).
  3. Ensure the launcher helper utility is installed (see last post).
  4. Set up environment variables requested in the bindings.
  5. Set up any launchers requested in the bindings.
  6. Build the argv for the new process and exec it.

OCaml

This is all very straight-forward:

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let execute_selections sels args config =
  let env = Env.copy_current_env () in
  let impls = make_selection_map config.Config.stores sels in
  let bindings = Binding.collect_bindings impls sels in

  ensure_runenv config;

  (* Do <environment> bindings *)
  List.iter (Binding.do_env_binding env impls) bindings;

  (* Do <executable-in-*> bindings *)
  List.iter (do_exec_binding config env impls) bindings;

  let command = ZI.get_attribute "command" sels in
  let prog_args = (Command.build_command impls (ZI.get_attribute "interface" sels) command env) @ args in
  Unix.execve (List.hd prog_args) (Array.of_list prog_args) (Env.to_array env);;

Haskell

This is rather complicated. Haskell functions can’t have side effects (like, say, creating a launcher or execing a process). Instead, the function returns an IO request to main, which returns it to whatever is driving Haskell. The IO request is basically a request to perform an operation and a callback to invoke when done. To avoid this becoming a syntax nightmare, Haskell’s do notation lets you write it in an imperative style:

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executeSelections :: Selections -> [Arg] -> Config -> IO ()
executeSelections sels userArgs config = do
		origEnv <- getEnvironment
		paths <- mapM resolvePath (toList $ selections sels)
		let pathMap = fromList $ catMaybes paths
		let env = foldl (doEnv pathMap) (fromList origEnv) bindings
		envWithExec <- doExecBindings config sels pathMap env bindings
		let argv = (buildCommand sels envWithExec pathMap (interface sels) commandName) ++ userArgs
		executeFile (head argv) False (tail argv) (Just $ toList envWithExec)
	where bindings = collectBindings sels
	      doEnv pathMap env (iface, binding) = doEnvBinding (Data.Map.lookup iface pathMap) env binding
	      Just commandName = (command sels)
	      resolvePath (iface, sel) = do mPath <- getPath config sel
	      				    case mPath of
						    Nothing -> return Nothing
						    Just path -> return $ Just (iface, path)

For example, take the first line: origEnv <- getEnvironment. getEnvironment is a request to get the current environment. It has the type IO [(String, String)] - a request for a list of (name, value) mappings. The rest of the do block is effectively a function which takes an argument origEnv, of type [(String, String)] (i.e. an actual list of mappings). When this is returned by main, the system gets the mappings and then calls this function with the results. The same thing then happens with the second line, and so on.

There’s one tricky bit: resolvePath takes a single selection and finds where it’s stored (which requires access to the filesystem). It returns an IO request to check whether some paths exist. But when we loop over all the selections, we get a list of IO operations, not an IO operation. So you need to use mapM (monadic map), which turns a list of IO requests into an IO request for a list. That’s a lot of thinking to do something quite simple.

Doing IO in Haskell is hard. Here’s another example (reading an environment variable):

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getEnvOpt :: VarName -> IO (Maybe String)
getEnvOpt var = do
	maybe_value <- tryIOError (getEnv var)
	case maybe_value of
		Left e -> if isDoesNotExistError e then return Nothing else ioError e
		Right ok -> return $ Just ok

I want to do a getEnv operation and return Nothing if the variable isn’t set. At first, I tried calling getEnv and catching the exception using the normal exception handing function. That doesn’t work. The reason is that getEnv doesn’t throw an exception; it successfully returns an IO request for an environment variable. You have to use the special tryIOError function to get a request for either an environment variable or an error, and then pattern-match on that.

The benefit of all this extra work is that you can instantly see which functions do (or rather request) IO by looking at their type signature. Take the XML parsing function, for example:

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parseXMLDoc :: String -> Maybe Element

Just by looking at the type, we can see that it does no IO (and, therefore, isn’t vulnerable to attacks which might try to trick it into loading local files while parsing the DTD). It also, incidentally, suggests that we’re not going the get any useful error message if it doesn’t parse: it will just return Nothing.

Data structures

We need to store the user’s configured search paths, following the XDG Base Directory Specification. We need a list of directories to search for configuration settings, cached data (e.g. downloads) and “data” (which we use mainly for binaries we’ve compiled).

Python

For storing general records, Python provides a choice of classes and tuples. Here’s a typical class and an example of creating an instance and accessing a field:

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class Basedirs:
	def __init__(self, data, cache, config):
		self.data = data
		self.cache = cache
		self.config = config

b = Basedirs(data = [".../share"],
	     cache = [".../cache"],
	     config = [".../config"])
print(b.data)

When calling any Python function or constructor, you can use the (optional) name = value syntax so you don’t have to remember the order of the arguments.

Python also provides named tuples, which saves some boilerplate code when you just want pure data with no methods attached. The syntax isn’t great, though, and you can’t do pattern matching on the names, only by remembering the order:

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from collections import namedtuple
Basedirs = namedtuple("Basedirs", ["data", "cache", "config"])
b = Basedirs(data = [".../share"],
	     cache = [".../cache"],
	     config = [".../config"])
print(b.data)

# Pattern matching
(one, two, three) = b
print(one)

OCaml

OCaml provides classes, unnamed tuples and records (essentially named tuples). A record would be the obvious choice here:

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type basedirs = {
  data: filepath list;
  cache: filepath list;
  config: filepath list;
};;

let b = {
  data = [".../share"];
  cache = [".../cache"];
  config = [".../config"];
};;

print_endline (String.concat "," b.data);;

Curiously, there’s no need to tell it the type of the records you’re building. It works it out from the field names. However, this does mean that you can’t define two different record types with the same field name in the same module. Also, if you want to access a field from a record defined in a different module, you need to qualify the field name, e.g.

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print_endline (String.concat "," b.Basedirs.data);;

There are also “polymorphic variants” which allow the same field name to be used in different structures, but I haven’t tried using them. The manual notes that the compiler can do a better job of finding errors with plain variants, however.

A big win over Python is the ability to pattern-match on field names. e.g. to extract the cache and config fields:

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let {cache; config} = b in
print_endline (String.concat "," cache);
print_endline (String.concat "," config)
;;

Update: if you compile with warnings on (and you should), it will complain that you’re ignoring the data field. This is a really useful check because if you add a new field later it will tell you all the places that might need updating. You can use data = _ to ignore that field explicitly, or just _ to ignore all remaining fields.

Haskell

Haskell doesn’t have classes (in the Python / OCaml sense), but it does provide named tuples:

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data Basedirs = Basedirs { share :: [FilePath]
			 , cache :: [FilePath]
			 , config :: [FilePath]
			 }

b = Basedirs { share = [".../share"]
	     , cache = [".../cache"]
	     , config = [".../config"]
             }

main = print $ show $ share b

Note that the syntax for accessing a field is field record, not record.field as in other languages. Like OCaml, you can’t use the same field name in different structures. Pattern matching works:

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main = do let Basedirs { cache = cacheDirs
                       , config = configDirs } = b
          print (show cacheDirs)
          print (show configDirs)

However, it doesn’t have OCaml’s short-cut for the common case where the field you want to match has the same name as the variable you want to store it in (to use Python terms). In fact, doing that is strongly discouraged, because if you matched with config = config, then that would shadow the config function used to access the record.

Variants

Sometimes, a value can be one of several sub-types. Bindings in 0install are a good example of this:

  • There are two types of binding: an EnvironmentBinding sets an environment variable to tell a program where some resource is, while an ExecutableBinding gives the program an executable launcher for another program.

  • An EnvironmentBinding can be used to find a path within the selected component, or to provide a constant value.

  • An EnvironmentBinding can affect its variable in three different ways: it can append the new value to the end of the old value, prepend it at the start, or replace the old value completely.

  • An ExecutableBinding can store the launcher’s location in a variable or add the launcher’s directory to the application’s $PATH.

Here’s an example of an environment binding which prepends a package’s lib directory to CLASSPATH:

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<environment name='CLASSPATH' insert='lib' mode='prepend'/>

The code that parses the XML and generates a list of bindings needs to store different values depending on which kind it is. For example, “append” and “prepend” bindings let you specify optional separator and default values, while “replace” ones don’t.

Then, the code that applies the bindings needs to handle each of the separate cases. We’d like to make sure that we didn’t forget to handle any case, and that we don’t try to access a field that’s only defined for a different case.

Python

The traditional object-oriented way to handle this is with subclasses (e.g. ExtendEnvironment with default and separator fields, and ReplaceEnvironment without; InsertPath and Value, etc). However, that’s a lot of classes to write, so 0install actually does everything in just two classes:

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class EnvironmentBinding(Binding):
	PREPEND = 'prepend'
	APPEND = 'append'
	REPLACE = 'replace'

	def __init__(self, name, insert, default = None, mode = PREPEND, value = None, separator = None):
		self.name = name
		self.insert = insert
		self.default = default
		self.mode = mode
		self.value = value
		if separator is None:
			self.separator = os.pathsep
		else:
			self.separator = separator
	...

class ExecutableBinding(Binding):
	IN_PATH = 'path'
	IN_VAR = 'var'

	def __init__(self, exec_type, name, command):
		self.exec_type = exec_type
		self.name = name
		self.command = command
	...

Note the use of strings (PREPEND, APPEND, REPLACE) in place of a proper enum, as Python doesn’t have them.

OCaml

Here’s a definition in OCaml. Variants use | to separate the various possibilities:

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type filepath = string;;
type varname = string;;

type which_end = Prepend | Append;;
type add_mode = {
	pos :which_end;
	default :string option;
	separator :string
};;
type mode = Add of add_mode | Replace;;
type env_source = InsertPath of filepath | Value of string;;
type env_binding = {
	var_name: varname;
	mode: mode;
	source: env_source
};;

type exec_type = InPath | InVar;;
type exec_binding = {
	exec_type: exec_type;
	name: string;
	command: string
};;

type binding =
  | EnvironmentBinding of env_binding
  | ExecutableBinding of exec_binding;;

That’s far more useful than the Python. It accurately describes the possible combinations, and is clear about the types and which bits are optional. Using varname and filepath as aliases for string doesn’t add any type safety, but it does make the signatures easier to read and gives better error messages.

Note that the extra | on the first line after type binding isn’t strictly necessary, but it helps to line things up.

Haskell

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type VarName = String

data WhichEnd = Prepend | Append deriving Show

data AddMode = AddMode { pos :: WhichEnd
		       , defaultValue :: Maybe String
		       , separator :: String
		       } deriving Show

data Mode = Add AddMode | Replace deriving Show

data EnvSource = InsertPath FilePath | Value String deriving Show

data ExecBindingType = InPath | InVar deriving Show

data Binding = EnvironmentBinding VarName Mode EnvSource
	     | ExecutableBinding ExecBindingType String String
	     deriving Show

This is essentially the same as the OCaml, except that I used tuples rather than records in Binding, because handling records is more awkward in Haskell due to the pattern matching problems noted above. Using tuples (which I could have done in OCaml too) makes the definitions shorter, because the definition of a tuple can be done in-line instead of with a separate structure.

deriving Show causes Haskell to automatically generate code to convert these types to strings, which is handy for debugging (and sadly missing from OCaml).

Using the data structures

To apply the bindings, a runner module needs to collect all the bindings and then:

  1. Process each EnvironmentBinding, updating the environment.
  2. Process each ExecutableBinding, using the new environment to create the launchers.

Here, we’ll look at the code to process an EnvironmentBinding.

Python

Here’s the Python code for applying an <environment> binding:

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class EnvironmentBinding(Binding):
	PREPEND = 'prepend'
	APPEND = 'append'
	REPLACE = 'replace'
	...
	def get_value(self, path, old_value):
		"""Calculate the new value of the environment variable after applying this binding.
		@param path: the path to the selected implementation; None for native packages
		@param old_value: the current value of the environment variable
		@return: the new value for the environment variable"""

		if self.insert is not None:
			if path is None:
				return None
			extra = os.path.join(path, self.insert)
		else:
			assert self.value is not None
			extra = self.value

		if self.mode == EnvironmentBinding.REPLACE:
			return extra

		if old_value is None:
			old_value = self.default
			if old_value is None:
				old_value = defaults.get(self.name, None)
				if old_value is None:
					return extra
		if self.mode == EnvironmentBinding.PREPEND:
			return extra + self.separator + old_value
		else:
			return old_value + self.separator + extra

The code for getting the value to append to is a bit messy. We’re trying to say:

  1. Use the current value of the environment variable. Or, if not set:
  2. Use the default from the <environment> element. Or, if not set:
  3. Use the built-in default value for this variable.

Python often makes this easy with its a or b or c syntax, but had to use the longer if syntax for this case because or treats the both None and the empty string as false, whereas we want to treat an empty string as a valid value.

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if old_value is None:
	old_value = self.default
	if old_value is None:
		old_value = defaults.get(self.name, None)

Actually, I noticed while writing this post that I got that wrong in the Python (I used the shorter or in one place) and had to fix it; a common mistake in Python.

The two binding types must be handled differently. In the run code, we (rather messily) check the type to decide what to do:

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if isinstance(binding, EnvironmentBinding):
	...
elif isinstance(binding, ExecutableBinding):
	...

This isn’t a very object-oriented way to do it. But it made more sense to put the logic for handling the bindings all together in the run module rather than in the module which defines the data types (which I prefer to keep side-effect free). Also, the rule that we need to process all the EnvironmentBindings before all of the ExecutableBindings can’t easily go in the classes themselves.

So, the existing Python code is really pretty poor. We’re using strings to simulate enums (simple variants), a single class with a load of if statements in place of variants for the different ways of setting an environment variable, and messy isinstance checks to let us keep the logic for applying bindings together in the run module. If we add or change binding classes, there’s several places we need to check, and no static checking to help us. Let’s see if the other languages can help us do better…

OCaml

Here’s the code to apply an EnvironmentBinding in OCaml:

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let calc_new_value name mode value env =
  match mode with
  | Replace -> value
  | Add {pos; default; separator} ->
    let add_to old = match pos with
      | Prepend -> value ^ separator ^ old
      | Append -> old ^ separator ^ value in
    match Env.find_opt name env with
      | Some v -> add_to v                  (* add to current value of variable *)
      | None -> match default with
        | Some d -> add_to d                (* or to the specified default *)
        | None -> match get_default name with
          | Some d -> add_to d              (* or to the standard default *)
          | None -> value                   (* no old value; use new value directly *)
;;

let do_env_binding env impls = function
| (iface, EnvironmentBinding {var_name; mode; source}) -> (
    let add value = Env.putenv var_name (calc_new_value var_name mode value env) env in
    match source with
    | Value v -> add v
    | InsertPath i -> match StringMap.find iface impls with
      | (_, None) -> ()  (* a PackageSelection; skip binding *)
      | (_, Some p) -> add (p +/ i)
)
| _ -> ()
;;

It’s not bad, and it’s nice to see all the different cases laid out.

By the way let do_env_binding env impls = function ... is a convenient way to pattern match on the last (unnamed) argument. It means the same as let do_env_binding env impls binding = match binding with ....

I initially thought that the Python version was easier to read. On reflection, however, I think it’s more subtle. It’s easier to see what the Python version does, but it’s not easy to see that what it does is correct. By contrast, it’s easy to see that the OCaml version handles every case (the compiler checks this), and you can just check that each individual case is handled correctly.

As in the Python, getting old_value is a bit messy as there’s no null coalescing operator in OCaml:

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match Env.find_opt name env with
  | Some v -> add_to v
  | None -> match default with
    | Some d -> add_to d
    | None -> match get_default name with
      | Some d -> add_to d
      | None -> value

Haskell

And here is the code to apply an EnvironmentBinding in Haskell:

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join :: WhichEnd -> String -> Maybe String -> String -> String
join _ _ Nothing new = new
join Prepend sep (Just old) new = new ++ sep ++ old
join Append sep (Just old) new = old ++ sep ++ new

doEnvBinding :: Maybe FilePath -> Env -> Binding -> Env
doEnvBinding mPath env (EnvironmentBinding name mode bindingSource) =
    case (mPath, bindingSource) of
	    (_, Value v) -> use v
	    (Nothing, InsertPath i) -> env
	    (Just implPath, InsertPath i) -> use $ implPath </> i
    where use newValue = insert name (add newValue) env
          add newValue = case mode of
	    Replace -> newValue
	    Add AddMode {pos = whichEnd, defaultValue = def, separator = sep} ->
		join whichEnd sep oldValue newValue
		where oldValue = (Data.Map.lookup name env) `mplus`
				 def `mplus` (standardDefault name)
doEnvBinding _ env _ = env

That’s a good bit shorter than both the Python and the OCaml, because I was able to use a `mplus` b handle the defaults easily. That’s “monadic plus”, in case you were wondering whether mplus is a bit of a silly name.

Handling XML

We need to parse the XML into some kind of internal representation. Originally, 0install parsed the XML into custom classes, but it turns out that we often want to write XML back out again, preserving attributes and elements we didn’t understand. So we’ve been slowly moving towards using generic XML trees as the in-memory representation, which saves having to write code to serialise our data structures as XML (which then requires ensuring that they’re consistent with the parsing code).

Python

Python’s standard library includes a selection of XML parsers: minidom, pulldom, ElementTree, expat and SAX. Disappointingly, the documentation says that none of them is safe with untrusted data. 0install uses the low-level “expat” parser, which isn’t in the vulnerabilities table, so hopefully we’re OK (many of the vulnerabilities are denial of service attacks, which isn’t a big problem for us).

We build our own “qdom” tree structure rather than use the more standard minidom module because import xml.dom.minidom takes too long to be used in this speed-critical code (or at least, it did when I wrote the qdom code, I haven’t tested it recently). One of the nice things about writing in the other languages is not having to worry about speed all the time.

OCaml

OCaml doesn’t include an XML parser in the standard libraries, so I Googled for one. The first one I found was Xml-Light, but it turned out that it didn’t support XML namespaces. Then I tried the PXP parser, which is enormous, and parsed the test document I gave it incorrectly (but they have fixed that now - thanks!). Finally, I tried Xmlm, which is small and works.

Xmlm doesn’t generate a tree structure, just events (like SAX), so you have to build your own structure. That could be a problem in some cases (it’s convenient to have a standard data structure in case you want to pass documents between modules). On the other hand, we already use our own document structure (“qdom”) in Python and the standard “DOM” interface is awkward anyway.

Xmlm suggests the following structure:

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type tree = E of Xmlm.tag * tree list | D of string

However, this is quite annoying to process, because every time someone gives you a tree you have to pattern match and handle the case of them giving you a D (a text node). Instead, I used ElementTree’s trick of attaching text to element nodes:

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type element = {
  tag: Xmlm.name;
  mutable attrs: Xmlm.attribute list;
  mutable child_nodes: element list;
  mutable text_before: string;        (** The text node immediately before us *)
  mutable last_text_inside: string;   (** The last text node inside us with no following element *)
};;

With this, it’s easy to iterate over all the elements and ignore text, and you don’t have to worry about getting two text nodes next to each other.

You generally don’t need text_before unless you’re using mixed content, but having it here means we don’t lose any data if we read a document and then write it out again.

It was convenient to have the structure be mutable while building it, and in other code we may want to manipulate nodes, so I marked most of the fields as mutable.

I made a serious mistake in my first attempt at pattern matching on elements. I wanted to match the elements <arg> or <for-each> in the 0install namespace:

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let xmlns_feed = "http://zero-install.sourceforge.net/2004/injector/interface";;

let process child = match child.Qdom.tag with
| (xmlns_feed, "arg") -> process_arg
| (xmlns_feed, "for-each") -> process_for_each
| _ -> ()

This is the worst kind of mistake: the kind that seems to work fine when you test it. I made the same mistake in Haskell, but luckily I decided to enable warnings (ghc -Wall) and spotted the problem. The code above doesn’t check that the namespace is equal to xmlns_feed. Instead, it creates a new xmlns_feed binding with whatever the namespace actually was. Obvious in hindsight (note: turning on warnings in OCaml also catches the mistake, because it complains that xmlns_feed is unused).

So, how can we fix this? Dealing with namespaces all the time when processing XML is annoying even when you get it right, so I decided to make a helper interface for doing queries in a particular namespace. Of course, I’d like this to generalise to other namespaces. I found an OCaml feature called “functors”, which seem to be basically module templates. Here’s how I made a namespace-specific query interface:

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module type NsType = sig
  val ns : string;;
end;;

module NsQuery (Ns : NsType) = struct
  (** Return the local name part of this element's tag.
      Returns [None] if it's in a different namespace. *)
  let tag elem =
    let (elem_ns, name) = elem.tag in
    if elem_ns = Ns.ns then Some name
    else None

  (** Apply [fn] to each child node in our namespace
      with local name [tag] *)
  let map fn node tag =
    let rec loop = function
      | [] -> []
      | (node::xs) ->
          if node.tag = (Ns.ns, tag)
          then (fn node) :: loop xs
          else loop xs in
    loop node.child_nodes
  ;;

  (** Get the value of the non-namespaced attribute [attr].
      Throws an exception if [elem] isn't in our namespace. *)
  let get_attribute attr elem =
  ...
end

Then I create a specialised version of this module for our namespace in constants.ml:

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module ZI_NS = struct
  let ns = "http://zero-install.sourceforge.net/2004/injector/interface";;
end;;

module ZI = Qdom.NsQuery (ZI_NS);;

With this, ZI.map applies a function to all elements in the 0install namespace and with the given name. ZI.tag returns the local name for 0install elements or Nothing for others, etc. Now the original code becomes:

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let process child = match ZI.tag child with
| Some "arg" -> process_arg
| Some "for-each" -> process_for_each
| _ -> ()

Much better!

Haskell

Again, there’s a choice of library. I went for Text.XML.Light which seems to work well.

Dealing with namespaces is fairly painful; I’m not convinced that I’m using it right. Here’s how I find the <runner> element inside a <command>, for example:

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getRunnerElement :: Element -> Maybe Element
getRunnerElement commandElem = filterChildName isRunner commandElem
	where isRunner qname = (qURI qname) == Just xmlns_feed && (qName qname) == "runner"

Using pattern matching doesn’t seem to improve things:

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getRunnerElement :: Element -> Maybe Element
getRunnerElement commandElem = filterChildName isRunner commandElem
	where isRunner (QName { qName = "runner", qURI = Just uri }) = uri == xmlns_feed
	      isRunner _ = False

I didn’t find any way to create a namespace-specific interface as in OCaml (Haskell “functors” do something different).

Testing it on malformed XML, it sometimes just returns Nothing, which is rather unhelpful, and sometimes it successfully parses it anyway! For reference, here is a test document that Haskell loads happily:

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<?xml version="1.0" ?>
<root a='h/>

Python and OCaml, by contrast, both detect the problem and report the location of the error.

Building lists

We need to walk the XML tree and return all the bindings, in document order. The places where a binding may be found are:

  • <selection> / [binding]
  • <selection> / [dependency] / [binding]
  • <selection> / <command> / [binding]
  • <selection> / <command> / [dependency] / [binding]

For bindings in a dependency, we need to know the dependency’s interface. For other bindings, we want the selection’s own interface.

Python

Python’s easy syntax for sets provides one way to do it, and its append method on lists makes it easy to collect the results:

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binding_elements = {'environment', 'executable-in-var', 'executable-in-path'}
dep_elements = {'requires', 'runner'}

def collect_bindings(impls, sels):
	bindings = []
	def process(elem, iface, valid_children):
		for child in elem.childNodes:
			if child.uri != ZI.ns: continue
			if child.name not in valid_children: continue

			if child.name == 'command':
				process(child, iface, binding_elements | dep_elements)
			elif child.name in dep_elements:
				dep_iface = child.attrs["interface"]
				if dep_iface in impls:
					process(child, dep_iface, binding_elements)
			elif child.name == 'environment':
				bindings.append((iface, EnvironmentBinding( ... ))
			elif child.name in ('executable-in-var', 'executable-in-path'):
				bindings.append((iface, ExecutableBinding( ... ))
			else:
				raise Exception(child.name)
	for sel in ZI.children(sels, "selection"):
		process(sel, sel.attrs["interface"], {'command'} | binding_elements | dep_elements)
	return bindings

I liked the ZI namespace query thing I made in OCaml so much, I made a Python class to do the same thing.

OCaml

I originally wrote this in a purely functional style, threading the results list through all the functions using fold_left. That was pretty messy. Then I decided to take advantage of OCaml’s support for imperative programming and just mutate a single list (bindings), as in the Python, which worked much better.

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let collect_bindings impls root =
  let bindings = ref [] in

  let rec process ~deps ~commands iface parent =
    let process_child node =
      match ZI.tag node with
      | Some "requires" | Some "runner" when deps ->
          let dep_iface = ZI.get_attribute "interface" node in
          if StringMap.mem dep_iface impls then process ~deps:false ~commands:false dep_iface node
          else ()
      | Some "command" when commands -> process ~deps:true ~commands:false iface node
      | _ -> match parse_binding node with
             | None -> ()
             | Some binding -> bindings := (iface, binding) :: !bindings in
    ZI.iter process_child parent in

  let process_sel node = process ~deps:true ~commands:true (ZI.get_attribute "interface" node) node in
  ZI.iter_with_name process_sel root "selection";
  List.rev !bindings
;;

Note the use of named arguments (process ~deps:true) to avoid confusion between the two boolean arguments. This works like process(deps = true) in Python.

A strange aspect of OCaml is that you generally write loops backwards, first declaring the code to handle each item and then calling an iter function to say what you want to loop over. OCaml does have a for loop, but it’s one of those old-fashioned BBC BASIC style ones that just updates a counter in that particular way that programmers never actually want to do.

Note also that in OCaml you add to the start of a list, not the end, so we need to reverse it as the last step.

Update: You can use the pipe operator (|>) to make loops easier to write. It lets you write the input to a function first:

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["one"; "two"; "three"] |> List.iter  (fun item ->
  print_endline item
)

Haskell

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collectBindings :: Selections -> [(InterfaceURI, Binding)]
collectBindings sels = do sel <- filterChildrenName (hasZName "selection") (root sels)
                          let iface = requireAttr "interface" sel
                          getBindings True True iface sel
  where getBindings deps commands iface parent =
            do (name, child) <- ziChildren parent
               case name of
                    "command" | commands -> getBindings True False iface child
                    x | (x == "requires" || x == "runner") && deps ->
                             let depIface = requireAttr "interface" child in
                             if depIface `member` (selections sels)
                               then getBindings False False depIface child
                               else []  -- Optional dependency which was not selected
                    _ -> do binding <- processBinding name child
                            [(iface, binding)]

Nice and short again, but again it requires some explanation. The do expressions here (unlike the previous one) have the list type, so they do the lines of the block in a way appropriate for lists. For lists, x <- items means to run the rest of the do block once for each item in the list items, producing a list for each one, and then join the resulting lists together end-to-end. So these do blocks are actually nested loops.

There are no named arguments, so we just have to remember what the True and False mean here.

Also, the | in the matches doesn’t separate alternatives (as in OCaml), but instead gives a condition which must also be true for it to match, like when in OCaml.

String processing

We need to expand environment variables in arguments. In the XML, it looks like this:

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<arg>-X${item}</arg>

Python

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from string import Template
value = Template("-X${item}").substitute({"item": "debug"})

I guess that’s cheating, since I picked Python’s preferred syntax when I designed the XML format. Here’s how to do it without using Template:

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import re
re_expand = re.compile(r"\$(\$|([a-zA-Z_][a-zA-Z0-9_]*)|{[^}]*})")
def expandArg(template, env):
	def expand_one(matched):
		match = matched.group(1)
		if match == '$': return '$'
		if match.startswith('{'): return env[match[1:-1]]
		return env[match]

	return re_expand.sub(expand_one, template)

Easy. Notice the handy r"..." syntax for raw strings, which avoids the need to escape backslashes everywhere.

Oh, and I seem to have written an OCaml-style backwards loop here. Guess they’re not unique to OCaml after all.

OCaml

The regex is looking a bit ugly, but still pretty good. Oddly, instead of getting some kind of MatchResult object passed to expand, we just get the original string and then pass that to the global match function. I guess that’s just how the underlying library works.

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let re_template = Str.regexp ("\\$\\(\\$\\|\\([a-zA-Z_][a-zA-Z0-9_]*\\)\\|{[^}]*}\\)")

let expand_arg template env =
  let remove_braces s =
    let l = String.length s in
    if s.[0] = '{' then (
      assert (s.[l - 1] = '}');
      String.sub s 1 (l - 2)
    ) else s; in
  let expand s = match (Str.matched_group 1 s) with
  | "$" -> "$"
  | "" | "{}" -> failwith ("Error: empty variable name in template: " ^ template)
  | m -> Env.find (remove_braces m) env in
  Str.global_substitute re_template expand template
;;

Haskell

This was hard, but stackoverflow to the rescue!

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expandArg :: String -> Env -> String
expandArg template env = replaceAll re replace template
	where re = makeRegex "\\$(\\$|([a-zA-Z_][a-zA-Z0-9_]*)|\\{[^\\}]*})"
	      replace match = case match of
	      			"$$" -> "$"
				'$' : '{' : ms -> replaceVar $ init ms
				'$' : var -> replaceVar var
				_ -> error "regex failed"
	      replaceVar var = case (Data.Map.lookup var env) of
	      				Nothing -> error (printf "Variable '%s' not set!" var)
					Just value -> value

-- Based on http://stackoverflow.com/a/9072362/50926
-- Use the given function to replace each occurance of 're' in 's'
replaceAll :: Regex -> (String -> String) -> String -> String
replaceAll re f s = prefix end
  where (_, end, prefix) = foldl' go (0, s, id) $ getAllMatches $ match re s
        go (ind,toRead,write) (off,len) =
          let (skip, start) = splitAt (off - ind) toRead
              (matched, remaining) = splitAt len start
          in (off + len, remaining, write . (skip++) . (f matched ++))

Finding resources

When shipping stand-alone binaries or 0install packages, it’s useful if the program can find other resources in its own code directory. In our case, we need to find the launcher program so we can symlink to it.

Python

In Python, this is really easy. Every module has an attribute __file__ which has the location of that module:

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my_dir = os.path.dirname(__file__)

OCaml

When called as a binary, we can get the path from argv[0] and use that. If we’re being used as a library then we’re either installed as a system package (and can look in some hard-coded path) or being run as a 0install dependency (and can get 0install to set an environment variable for us). Easy enough, but abspath (and realpath) are missing from the standard library.

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let abspath path =
  if path.[0] = '/' then path
  else (Sys.getcwd ()) +/ path
;;

my_dir = Filename.dirname (abspath Sys.argv.(0))

Haskell

Wow. I have no idea how this works.

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{-# LANGUAGE ForeignFunctionInterface #-}
module Main where

import Foreign
import Foreign.C

import System.Directory (getCurrentDirectory)

main :: IO ()
main = do progPath <- getFullProgName
	  absProgPath <- absolute_path progPath
	  let myDir = dropFileName absProgPath
	  ...

-- From http://hackage.haskell.org/trac/ghc/ticket/3199
getFullProgName :: IO String
getFullProgName =
  alloca $ \ p_argc ->
  alloca $ \ p_argv -> do
   getFullProgArgv p_argc p_argv
   peek p_argv >>= peek >>= peekCString

foreign import ccall unsafe "getFullProgArgv"
    getFullProgArgv :: Ptr CInt -> Ptr (Ptr CString) -> IO ()

-- From http://hackage.haskell.org/packages/archive/MissingH/1.2.0.0/doc/html/System-Path-NameManip.html
absolute_path :: String -> IO String
absolute_path path@('/':_) = return path
absolute_path path = do
   cwd <- getCurrentDirectory
   return (cwd ++ "/" ++ path)

API docs

All three languages make it easy to generate cross-referenced API docs. I didn’t bother to fill in many strings for the OCaml and Haskell, but the linked sample pages show what it looks like:

Python and Haskell require you to document the types in the code if you want them to appear in the docs. The Python syntax for this is a bit awkward. OCaml infers the types automatically.

Speed

My test case is running 0release (which has the largest selections.xml of all my apps), except that I edited the selections so it runs /bin/echo at the end rather than /usr/bin/python2. Otherwise we’re just measuring the speed of Python. Running echo directly with a trivial (hard-coded) C program takes 2-3 ms, so that’s the theoretical best time.

Language Time / ms Lines of code Non-blank chars
(baseline) 2 - -
OCaml 7 678 17,201
Haskell 14 543 16,818
Python (test) 45 / 76 576 14,685
Python (real) 64 / 109 21,427 609,082

Enabling stack traces in OCaml (OCAMLRUNPARAM=b) increased the time from 7ms to 8ms.

“Python (test)” is a version I wrote for this comparison to get a feel for the number of lines. It doesn’t do as much checking as the real version and lacks some features. “Python (real)” is the regular “0launch” command. The first number is the time with Python 2, the second with Python 3. It’s interesting how much faster the test version is!

By the way, if you’re thinking “parsing XML just to set up environment variables is inefficient; I’ll just use a shell script to start my program”, a /bin/sh script which just execs “echo” takes 10 ms.

Conclusions

All three languages are easy to read, avoiding verbose boilerplate code, and ending up fairly similar in length. No doubt an expert in OCaml or Haskell would write much shorter code than I did. OCaml’s syntax is simple, but the indentation doesn’t necessarily match up with OCaml’s interpretation of the code, which is a concern. In Python and Haskell, the compiler always sees the same structure as the programmer.

Haskell provides all kinds of special notations for writing shorter code. This means that reading other people’s Haskell code can be very difficult. Also, the same structure (e.g. do) can mean wildly different things in different contexts. OCaml syntax is easy to learn and easy to understand.

Many things that are simple in Python or OCaml become very complex in Haskell. In this example: reading an environment variable that may be unset, doing any kind of IO, reporting errors in XML documents, search-and-replace in strings and reading argv. And although not part of this test, I think the converting 0install’s solver to Haskell would be very difficult.

Haskell’s ability to control where IO happens is useful, but other languages (e.g. E and (hopefully) OCaml with Emily) achieve the same thing without all the complexity.

Python’s data structures are very limited and error prone. OCaml and Haskell record and variant types significantly improve the code. OCaml makes working with record types easier, while Haskell simplifies debugging by making it easy to convert structures to strings.

XML handling was easiest in OCaml, though I did have to make my own tree structure. Python’s XML libraries aren’t safe and Haskell’s doesn’t report (or even detect, in some cases) errors.

Walking the XML tree and building a list was easy in all three languages. In Python and OCaml, because they let us mutate a single list while walking the tree, and in Haskell thanks to its List monad which handles everything behind the scenes.

A major benefit of OCaml and Haskell is the ease of refactoring. In Python, once the code is working it’s best not to change things, in case you break something that isn’t covered by the unit-tests. In OCaml and Haskell, you can rename a function, delete old code or change a data structure and rely on the compiler to check that everything’s still OK. The static typing in OCaml and Haskell also gives me more confidence that that various untested error reporting code paths will actually work. Untested Python code is buggy code, generally.

I’m not sure how stable the OCaml library APIs are (I assume they don’t change much), but it’s clear that Haskell’s APIs change frequently: code samples and documentation I found on the ‘net were frequently out of date and wouldn’t compile without changes. Python is generally pretty good, if you overlook the massive changes in Python 3.

In the previous (trivial) test, OCaml and Haskell had very similar performance, but here OCaml clearly moves into the lead. Python is far behind, and only getting slower with the move to Python 3. Being able to write code without worrying about speed all the time is very liberating!

The two statically typed languages didn’t require much debugging, except that OCaml’s find function throws an exception if the key isn’t found rather than using an option type (as in Haskell), which can lead to non-obvious errors. Turning on stack-traces in OCaml makes it easy to track these down however, and making my own wrappers for find and similar should mean it won’t be a problem in general.

The big surprise for me in these tests was how little you lose going from Python to OCaml. You still have classes, objects, functions, mutable data and low-level access to the OS, all with an easy and concise syntax, but you gain type checking, much better data structures and a huge amount of speed for no additional effort. Why aren’t more people using it?

Although Haskell and OCaml look more similar to each other than to Python, this is just syntax. In fact, OCaml and Python are conceptually pretty similar, while Haskell is different in almost every way.

Overall, binary compatibility (i.e. difficulty of making cross-platform releases of the library, which is currently easy with Python) is my main remaining concern. But while further testing is required (particularly for networking, threading and GUI support), so far I’d be happy to move to OCaml.

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