Execing Processes in Haskell

Here’s the translation of the Go code to Haskell, along with explanations in Markdown format suitable for Hugo:

In this example, we’ll look at executing external processes in Haskell. Sometimes we want to completely replace the current Haskell process with another (perhaps non-Haskell) one. To do this, we’ll use Haskell’s implementation of the classic exec function.

import System.Process
import System.Environment
import System.Posix.Process

main :: IO ()
main = do
    -- For our example we'll exec `ls`. We'll use `findExecutable` to find 
    -- the absolute path to the binary we want to execute (probably `/bin/ls`).
    maybeBinary <- findExecutable "ls"
    case maybeBinary of
        Nothing -> error "ls command not found"
        Just binary -> do
            -- `exec` requires arguments in list form. We'll give `ls` a few
            -- common arguments. Note that the first argument should be the program name.
            let args = ["ls", "-a", "-l", "-h"]

            -- We also need a set of environment variables to use.
            -- Here we just provide our current environment.
            env <- getEnvironment

            -- Here's the actual `executeFile` call. If this call is successful,
            -- the execution of our process will end here and be replaced by
            -- the `/bin/ls -a -l -h` process. If there is an error, it will be thrown.
            executeFile binary True args (Just env)

When we run our program, it is replaced by ls:

$ runhaskell execing-processes.hs
total 16K
drwxr-xr-x  4 user user 4.0K Oct 3 16:29 .
drwxr-xr-x 91 user user 4.0K Oct 3 12:50 ..
-rw-r--r--  1 user user 1.3K Oct 3 16:28 execing-processes.hs

Note that Haskell, like many high-level languages, does not offer a classic Unix fork function. However, this isn’t usually an issue, as Haskell’s concurrency features and the ability to spawn and exec processes cover most use cases for fork.

In this Haskell version:

1. We use the System.Process module to find the executable and execute it.
2. findExecutable is used instead of exec.LookPath to find the full path of the ls command.
3. We use executeFile from System.Posix.Process instead of syscall.Exec. This function replaces the current process with the new one.
4. Error handling is done using Haskell’s Maybe type and pattern matching, rather than explicit error checks.
5. The environment is obtained using getEnvironment from System.Environment.

This example demonstrates how to replace the current Haskell process with another executable, which is analogous to the exec system call in Unix-like operating systems.