# Stack's script interpreter 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 ` 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: ~~~haskell #!/usr/bin/env stack -- stack script --resolver lts-21.13 --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: ~~~text 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 21.13 snapshot (`--resolver lts-21.13`) and ensuring the [`turtle` package](https://hackage.haskell.org/package/turtle) is available (`--package turtle`). The version of the package will be that in the specified snapshot (`lts-21.13` provides `turtle-1.6.1`). ## Arguments and interpreter options and arguments Arguments for the script can be specified on the command line after the file name: `stack ...`. 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 MyScript.hs ` with Stack interpreter options comment: ~~~haskell -- stack -- ~~~ is equivalent to the following command at the command line: ~~~text stack -- MyScript.hs ~~~ 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`: ~~~haskell #!/usr/bin/env stack {- stack script --resolver lts-21.13 -- +RTS -s -RTS -} import Data.List (intercalate) import System.Environment (getArgs) import Turtle (echo, fromString) main = do args <- getArgs echo $ fromString $ intercalate ", " args ~~~ is equivalent to the following command at the command line: ~~~text stack script --resolver lts-21.13 -- MyScript.hs arg1 arg2 +RTS -s -RTS ~~~ where `+RTS -s -RTS` are some of GHC's [runtime system (RTS) options](https://downloads.haskell.org/~ghc/latest/docs/users_guide/runtime_control.html). ## Just-in-time compilation 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.) ## Using multiple packages 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: ~~~haskell #!/usr/bin/env stack {- stack script --resolver lts-21.13 --package turtle --package "stm async" --package http-client,http-conduit -} ~~~ ## Stack configuration for scripts 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 `/global-project/stack.yaml`. ## Testing scripts 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`](https://hackage.haskell.org/package/acme-missiles): ~~~haskell {- stack script --resolver lts-21.13 --package acme-missiles -} import Acme.Missiles (launchMissiles) main :: IO () main = launchMissiles ~~~ The command `stack --script-no-run-compile Script.hs` then behaves as if the command `stack script --resolver lts-21.13 --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! ## Writing independent and reliable scripts The `stack script` command will automatically: * Install GHC and libraries, if missing. `stack script` behaves as if the `--install-ghc` flag 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: 1. Use the `--install-ghc` option to install the compiler automatically 2. Explicitly specify all packages required by the script using the `--package` option. Use `-hide-all-packages` GHC option to force explicit specification of all packages. 3. Use the `--resolver` Stack 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`: ~~~haskell #!/usr/bin/env stack {- stack runghc --install-ghc --resolver lts-21.13 --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. ## Loading scripts in GHCi 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: ~~~haskell #!/usr/bin/env stack {- stack exec ghci --install-ghc --resolver lts-21.13 --package turtle -} ~~~