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How to Script with Stack

This tutorial assumes you’ve already installed the Stack build tool. If you haven’t yet, please start with the get started page and then come back here.

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In this article we’ll be talking about how to write short scripts and self-contained programs in Haskell with Stack.

Accessing the Haskell tools

When you install Stack, the common Haskell tools like ghc and ghci do not end up on your PATH. This is to allow you to easily switch between different GHC versions on projects. The easiest way to access these tools is to prefix any command with stack exec --, e.g.:

stack exec -- ghc --version

If you haven’t installed GHC yet, you’ll get an error message about ghc not being found. You can resolve that by running stack setup first, or by adding the --install-ghc option to enable automatic GHC installation:

stack setup
# or
stack --install-ghc exec -- ghc --version

NOTE The -- after exec tells Stack to pass the rest of the command line arguments to the specified program instead of parsing them itself.

Compiling an executable

Let’s write a simple program, save it in HelloWorld.hs:

main :: IO ()
main = putStrLn "Hello World!"

To compile it, you can run

$ stack exec -- ghc HelloWorld.hs

This uses the ghc executable directly to perform the compilation. But actually, since exec ghc is such a common combination, we have a convenience shortcut available:

$ stack ghc -- HelloWorld.hs

You can now run your program with ./HelloWorld, or on Windows HelloWorld.exe.

Running a script

We don’t always want to compile an executable. Sometimes it’s convenient to just use GHC’s scripting capabilities. To do that, you can run:

$ stack exec -- runghc HelloWorld.hs

Or, as you may have guessed:

$ stack runghc -- HelloWorld.hs

Adding packages

Let’s say we want to use a package that doesn’t ship with GHC itself, like http-conduit. The stack exec command – and its shortcuts like stack ghc and stack runghc – all take a --package argument indicating that an additional package needs to be present. So consider this http.hs file:

{-# LANGUAGE OverloadedStrings #-}
import qualified Data.ByteString.Lazy.Char8 as L8
import           Network.HTTP.Simple

main :: IO ()
main = do
    response <- httpLBS "https://httpbin.org/get"

    putStrLn $ "The status code was: " ++
               show (getResponseStatusCode response)
    print $ getResponseHeader "Content-Type" response
    L8.putStrLn $ getResponseBody response

You can ensure that it runs with the http-conduit package available by using:

$ stack runghc --package http-conduit -- http.hs

Specify a resolver

There’s unfortunately a major downside to everything we’ve seen so far: there are no guarantees given about which version of GHC or libraries will be used. The selection will depend entirely on what Stack deems the most appropriate resolver – or collection of GHC and libraries – when you start using it. If you want to be guaranteed that a specific set of packages will be used, you can set the --resolver on the command line.

$ stack --resolver lts-12.21 runghc --package http-conduit http.hs

Script interpreter

Remembering to pass all of these flags on the command line is very tedious, error prone, and makes it difficult to share scripts with others. To address this, Stack has a script interpreter feature which allows these flags to be placed at the top of your script. Stack also has a dedicated script command which has some nice features like auto-detection of packages you need based on import statements.

If we modify our http.hs to say:

#!/usr/bin/env stack
-- stack --resolver lts-12.21 script
{-# LANGUAGE OverloadedStrings #-}
import qualified Data.ByteString.Lazy.Char8 as L8
import           Network.HTTP.Simple

main :: IO ()
main = do
    response <- httpLBS "https://httpbin.org/get"

    putStrLn $ "The status code was: " ++
               show (getResponseStatusCode response)
    print $ getResponseHeader "Content-Type" response
    L8.putStrLn $ getResponseBody response

We can now run it by simply typing:

$ stack http.hs

Furthermore, on POSIX systems, that line at the top is known as the shebang. This means that, if you make your script executable, you can just run it like a normal program:

$ chmod +x http.hs
$ ./http.hs

If you want to create self contained scripts, a script interpreter line at the top of your file is a highly recommended practice.

What’s next

The intuitions we’ve built for the ghc and runghc commands applies equally well to the ghci REPL. This is discussed at more length in the How to Play tutorial. The quick summary is: stack exec --package http-conduit -- ghci to get a powerful REPL ready to make HTTP requests.

While the script interpreter is great for small programs, it doesn’t scale nicely to very large projects. At that point, we recommend you bump up to a full-blown Stack project, which allows for more complex configuration via a stack.yaml file. You can find more details on this in How to Build.

There’s also a blog post available on writing bitrot-free scripts with Haskell, which heavily uses the techniques mentioned here.

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