Introduction

I've implemented a couple Unix core utilities in Haskell, and want to continue a series of posts going through the details - starting with simple programs like cat , seq , and which , and then moving on towards more featureful programs like uniq , tr and maybe grep .

So, let's implement which in Haskell!

Background

On most operating systems (Linux, Windows, MacOS, *BSD), the PATH environment variable defines which directories contain executables. which helps you find an executable by searching through these directories.

On Linux, PATH typically looks something like this:

% echo $PATH / usr / local / sbin : / usr / local / bin : / usr / sbin : / usr / bin : / sbin : / bin : / usr / games : / usr / local / games : / sbin : / usr / local / sbin : / usr / sbin

A colon separated, ordered list of directory paths.

On Windows, PATH looks like this

PS C : \ > echo $ Env : PATH C : \ Windows \ system32 ; C : \ Windows ; C : \ WindowsSystem32 \ Wbem ; C : \ Windows \ System32 \ WindowsPowerShell \ v1 . 0 ; C : \ Users \ leaf \ AppData \ Roaming \ local \ bin

A semicolon separated, ordered list of directory paths.

The goal is the same for both, parse the variable output into a list, search each of these directories in order, and return the first match. This is mirroring the behavior that your shell ( sh , zsh , powershell ) performs to determine what executable to run based on the command names you provide.

It's reasonable for executables with the same name to appear in different places in PATH , which makes the ordering important. For instance, if you had a custom cat executable on Linux, you could place that in /usr/local/bin , and it would shadow the system copy in /bin .

Module and Imports

The top of the file contains the module definition and imports. Since this is going to be an executable, not a library, we use module Main where . We'll skip going through the imports for now, but reference them as we move through the file. To follow along with the examples, you can put this header in a file and load it from ghci with :load which.hs . This GitHub repository has a working .cabal file as an example.

module Main where -- which -- -- look for the arguments on $PATH import Control.Monad ( filterM , when ) import Data.List.Split ( splitOn ) import Data.Maybe import System.Directory import System.Environment ( getArgs , getEnv ) import System.Exit ( exitFailure ) import System.Info ( os )

Data Flow

Our input comes from two places, the command line and the environment. PATH expands to a list of directories that may contain the target file. We need to maintain the following constraints:

only consider directories on PATH that exist

that exist there must be a file with the same name in the directory

the file match we found must be executable

if no match can be found, continue, but exit with failure when finished

After expanding PATH , we'll be able to use filters to apply each of the constraints.

Parsing PATH

Lets start with parsing PATH . To read the variable, we need two functions to start:

getEnv :: String -> IO String

splitOn :: Eq a => [a] -> [a] -> [[a]]

Composing these with <$> shows we're on the right track - dividing PATH into directories.

* Main > take 5 . reverse . splitOn ":" <$> getEnv "PATH" [ "/usr/local/games" , "/usr/games" , "/bin" , "/sbin" , "/usr/bin" ]

What if some of these directories are invalid? Or point to a network location that's not accessible right now? We need to filter out directories that don't exist. Since checking for directory existance is necessarily dependant on the outside world, we'll need filterM instead of filter .

* Main > take 5 . reverse . splitOn ":" <$> getEnv "PATH" >>= filterM doesDirectoryExist [ "/usr/local/games" , "/usr/games" , "/bin" , "/sbin" , "/usr/bin" ]

This works for Linux, but what about Windows? We need some platform dependant behavior and a function to wrap this up. The os constant from System.Info gives us a plain String to work with. Alternatively, we could use the CPP language extension and #ifndef mingw32_HOST_OS to make this decision at compile time. For this kind of simple difference, keeping the behavior present in the code seems cleaner.

getPaths :: IO [ FilePath ] -- ^ convert the PATH variable to a list of valid directories getPaths = splitOn separator <$> getEnv "PATH" >>= filterM doesDirectoryExist where separator = case os of "mingw32" -> ";" _ -> ":"

vs

getPaths :: IO [ FilePath ] -- ^ convert the PATH variable to a list of valid directories getPaths = splitOn separator <$> getEnv "PATH" >>= filterM doesDirectoryExist where # ifndef mingw32_HOST_OS separator = ":" -- POSIX # else separator = ";" -- Windows # endif

Filtering on IO

Now that we have a list of directories to consider, we can start restricting ( filter ) and manipulating ( map ) the list to match the constraints. The map and filter functions apply non-Monad functions to iterables. This is what we're used to when working with pure code.

-- even :: Integral a => a -> Bool -- map (*2) :: Num b => [b] -> [b] * Main > map ( * 2 ) . filter even $ [ 1 .. 10 ] [ 4 , 8 , 12 , 16 , 20 ]

Our input comes from the IO Monad, but this isn't a problem. We can use <$> to apply pure functions on Monad values, like we did to split the PATH variable. The tricky part is that the functions we need to apply for filtering and mapping produce IO values.

executable :: FilePath -> IO Bool doesFileExist :: FilePath -> IO Bool

mapM and filterM allow us to apply these functions, and >>= lets us bind them together, similar to what we did with . and map + filter .

filterM :: Applicative m => ( a -> m Bool ) -> [ a ] -> m [ a ] mapM :: ( Traversable t , Monad m ) => ( a -> m b ) -> t a -> m ( t b )

The flow of data will be:

a list of directories

a list of filepaths (directory + file)

a list of filepaths that exist

a list of executable filepaths

Translating this flow into Haskell:

search :: String -> IO [ FilePath ] -- ^ look for a file in the right directories, that's executable -- instead of checking dir contents, create the path and check for existence search file = getPaths >>= mapM addFileToPath >>= filterM doesFileExist >>= filterM runnable where addFileToPath directory = pure $ pathJoin directory file

With this framework in place, the next step is to fill in the supporting functions to do the real world work. pathJoin acts like Python's os.path.join() - be platform aware and don't add unnecessary path separators. We can't guarantee that the path names end in separators. For example, /usr/bin vs /usr/bin/ .

runnable :: FilePath -> IO Bool -- ^ is this file executable? expects it to exist runnable file = executable <$> getPermissions file pathJoin :: FilePath -> String -> FilePath -- ^ join a directory path and filename, platform aware -- could have used System.FilePath.Posix (filePath) here pathJoin [] file = file pathJoin base file = if last base == separator then base <> file else base <> [ separator ] <> file where separator = case os of "mingw32" -> ' \\ ' _ -> '/'

Putting it all together

The hard work is done, what's left to do is tie it all together. The error condition is when no match could be found. Otherwise, we print the first result. head and Maybe provide a simple way to express this.

main :: IO () -- ^ print the path to each argument if possible -- if anything didn't exist, exit failure main = do paths <- getArgs >>= mapM which when ( any isNothing paths ) exitFailure mapM_ putStrLn $ catMaybes paths which :: String -> IO ( Maybe String ) -- ^ grab the first result, if there was one which file = maybeHead <$> search file where maybeHead [] = Nothing maybeHead ( x : _ ) = Just x

main applies each command line argument to this function, and checks for Nothing .

Conclusion

We have a fully functional, platform independent implementation of which in Haskell! We used filterM and mapM to work with IO functions in an elegant way, and produce a pipeline that applies each constraint. System.Info (os) let's us account for differences in platforms. Moreover, it takes advantage of Haskell's lazy evaluation, and won't continue searching for a match after it's been found.

Linux

$ stack exec which python bash /usr/bin/python /bin/bash

Windows

PS C:\> stack exec which python.exe cmd.exe C:\Users\leaf\AppData\Local\Programs\Python\Python37-32\python.exe C:\Windows\system32\cmd.exe

Thanks to u/andersk for pointing some efficiency improvements!

Full implementation

Here's the full source. You can also find it here.