Stack offers a very useful feature for running files: a script interpreter. For too long have Haskellers felt shackled to bash or Python because it's just too hard to create reusable source-only Haskell scripts. Stack attempts to solve that.
You can use stack <file_name> to execute a Haskell source file. Usually, the
Stack command to be applied is specified using a special Haskell comment (the
Stack interpreter options comment) at the start of the source file. That command
is most often stack script but it can be, for example, stack runghc. If
there is no Stack interpreter options comment, Stack will warn that one was
expected.
An example will be easiest to understand. Consider the Haskell source file
turtle-example.hs with contents:
#!/usr/bin/env stack
-- stack script --snapshot lts-22.21 --package turtle
{-# LANGUAGE OverloadedStrings #-}
import Turtle (echo)
main = echo "Hello World!"=== "Unix-like"
The first line beginning with the 'shebang' (`#!`) tells Unix to use Stack
as a script interpreter, if the file's permissions mark it as executable. A
shebang line is limited to a single argument, here `stack`.
The file's permissions can be set with command `chmod` and then it can be
run:
~~~text
chmod +x turtle-example.hs
./turtle-example.hs
~~~
!!! note
On macOS:
- Avoid `{-# LANGUAGE CPP #-}` in Stack scripts; it breaks the shebang
line ([GHC #6132](https://gitlab.haskell.org/ghc/ghc/issues/6132))
- Use a compiled executable, not another script, in the shebang line.
Eg `#!/usr/bin/env runhaskell` will work but
`#!/usr/local/bin/runhaskell` would not.
Alternatively, the script can be run with command:
~~~text
stack turtle-example.hs
~~~
=== "Windows (with PowerShell)"
The first line beginning with the 'shebang' (`#!`) has a meaning on
Unix-like operating systems but will be ignored by PowerShell. It can be
omitted on Windows. The script can be run with command:
~~~text
stack turtle-example.hs
~~~
In both cases, the command yields:
Hello World!
the first time after a little delay (as GHC is downloaded, if necessary, and dependencies are built) and subsequent times more promptly (as the runs are able to reuse everything already built).
The second line of the source code is the Stack interpreter options comment. In
this example, it specifies the stack script command with the options of a
LTS Haskell 22.21 snapshot (--snapshot lts-22.21) and ensuring the
turtle package is available
(--package turtle). The version of the package will be that in the specified
snapshot (lts-22.21 provides turtle-1.6.2).
Arguments for the script can be specified on the command line after the file
name: stack <file_name> <arg1> <arg2> ....
The Stack interpreter options comment must specify what would be a single valid
Stack command at the command line if the file name were included as an argument,
starting with stack. It can include -- followed by arguments. In particular,
the Stack command stack <arg1> MyScript.hs <arg4> with
Stack interpreter options comment:
-- stack <arg2> <command> <arg3> -- <arg5>is equivalent to the following command at the command line:
stack <arg1> <arg2> <command> <arg3> -- MyScript.hs <arg4> <arg5>
The Stack interpreter options comment must be the first line of the file, unless a shebang line is the first line, when the comment must be the second line. The comment must start in the first column of the line.
When many options are needed, a block style comment that splits the command over more than one line may be more convenient and easier to read.
For example, the command stack MyScript.hs arg1 arg2 with MyScript.hs:
#!/usr/bin/env stack
{- stack script
--snapshot lts-22.21
--
+RTS -s -RTS
-}
import Data.List (intercalate)
import System.Environment (getArgs)
import Turtle (echo, fromString)
main = do
args <- getArgs
echo $ fromString $ intercalate ", " argsis equivalent to the following command at the command line:
stack script --snapshot lts-22.21 -- MyScript.hs arg1 arg2 +RTS -s -RTS
where +RTS -s -RTS are some of GHC's
runtime system (RTS) options.
As with using stack script at the command line, you can pass the --compile
flag to make Stack compile the script, and then run the compiled executable.
Compilation is done quickly, without optimization. To compile with optimization,
pass the --optimize flag instead. Compilation is done only if needed; if the
executable already exists, and is newer than the script, Stack just runs the
executable directly.
This feature can be good for speed (your script runs faster) and also for durability (the executable remains runnable even if the script is disturbed, eg due to changes in your installed GHC/snapshots, changes to source files during git bisect, etc.)
As with using stack script at the command line, you can also specify multiple
packages, either with multiple --package options, or by providing a comma or
space separated list. For example:
#!/usr/bin/env stack
{- stack script
--snapshot lts-22.21
--package turtle
--package "stm async"
--package http-client,http-conduit
-}With the stack script command, all Stack YAML configuration files (global and
project-level) are ignored.
With the stack runghc command, if the current working directory is inside a
project then that project's Stack project-level YAML configuration is effective
when running the script. Otherwise the script uses the global project
configuration specified in <Stack root>/global-project/stack.yaml.
You can use the flag --script-no-run-compile on the command line to enable (it
is disabled by default) the use of the --no-run option with stack script
(and forcing the --compile option). The flag may help test that scripts
compile in CI (continuous integration).
For example, consider the following simple script, in a file named Script.hs,
which makes use of the joke package
acme-missiles:
{- stack script
--snapshot lts-22.21
--package acme-missiles
-}
import Acme.Missiles (launchMissiles)
main :: IO ()
main = launchMissilesThe command stack --script-no-run-compile Script.hs then behaves as if the
command
stack script --snapshot lts-22.21 --package acme-missiles --no-run --compile -- Script.hs
had been given. Script.hs is compiled (without optimisation) and the resulting
executable is not run: no missiles are launched in the process!
The stack script command will automatically:
- Install GHC and libraries, if missing.
stack scriptbehaves as if the--install-ghcflag had been passed at the command line. - Require that all packages used be explicitly stated on the command line.
This ensures that your scripts are independent of any prior deployment specific configuration, and are reliable by using exactly the same version of all packages every time it runs so that the script does not break by accidentally using incompatible package versions.
In earlier versions of Stack, the runghc command was used for scripts and can
still be used in that way. In order to achieve the same effect with the runghc
command, you can do the following:
- Use the
--install-ghcoption to install the compiler automatically - Explicitly specify all packages required by the script using the
--packageoption. Use-hide-all-packagesGHC option to force explicit specification of all packages. - Use the
--snapshotStack option to ensure a specific GHC version and package set is used.
It is possible for configuration files to affect stack runghc. For that
reason, stack script is strongly recommended. For those curious, here is an
example with runghc:
#!/usr/bin/env stack
{- stack
runghc
--install-ghc
--snapshot lts-22.21
--package base
--package turtle
--
-hide-all-packages
-}The runghc command is still very useful, especially when you're working on a
project and want to access the package databases and configurations used by that
project. See the next section for more information on configuration files.
Sometimes you want to load your script in GHCi to play around with your program.
In those cases, you can use exec ghci option in the script to achieve
it. Here is an example:
#!/usr/bin/env stack
{- stack
exec ghci
--install-ghc
--snapshot lts-22.21
--package turtle
-}