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script_command.md

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The stack script command

stack script [--package PACKAGE] FILE
             [-- ARGUMENT(S) (e.g. stack script X.hs -- argument(s) to program)]
             [--compile | --optimize] [--[no-]use-root] [--ghc-options OPTIONS]
             [--extra-dep PACKAGE-VERSION] [--no-run]

The stack script command either runs a specified Haskell source file (using GHC's runghc) or, optionally, compiles such a file (using GHC) and, by default, runs it.

Unlike stack ghc and stack runghc, the command ignores all Stack YAML configuration files (global and project-level). A snapshot must be specified on the command line (with the --snapshot option). For example:

stack script --snapshot lts-22.21 MyScript.hs

The stack script command behaves as if the --install-ghc flag had been passed at the command line.

Everything after -- on the command line is interpreted as a command line argument to be passed to what is run.

A package can be added to the snapshot on the command line with the --extra-dep option (which can be specified multiple times).

Each required package can be specified by name on the command line with the --package option (which can be specified multiple times). A single --package option can also refer to a list of package names, separated by a space or comma character. If the package is not in the snapshot, the most recent version in the package index (e.g. Hackage) will be obtained.

If no packages are specified in that way, all the required packages that are in the snapshot or are a GHC boot package (packages that come with GHC and are included in GHC's global package database) will be deduced by reference to the import statements in the source file. The base package associated with the version of GHC specified by the snapshot is always available.

If a required package is a GHC boot package, the behaviour can be complex. If the boot package has not been 'replaced', then it will be used in Stack's build plan. However, if the boot package has been 'replaced', the latest version of that package in the package index will be used in Stack's build plan, which may differ from the version provided by the version of GHC specified by the snapshot. A boot package will be treated as 'replaced' if the package i included directly in the Stackage snapshot or it depends on a package included directly in the snapshot. Stackage snapshots do not include directly most boot packages but some snapshots may include directly some boot packages. In particular, some snapshots include directly Win32 (which is a boot package on Windows) while others do not. For example, if Cabal (a boot package) is a required package then, with Stackage snapshot LTS Haskell 20.25, Stack will:

  • on Windows, try to construct a build plan based on the latest version of Cabal in the package index (because that snapshot includes Win32 directly, and Cabal depends on Win32 and so is treated as 'replaced'); and
  • on non-Windows, use the boot package in the build plan (because Cabal is not 'replaced').

Boot packages that have been 'replaced' can be specified as an --extra-dep.

The source file can be compiled by passing either the --compile flag (no optimization) or the --optimize flag (compilation with optimization). If the file is compiled, passing the --no-run flag will mean the compiled code is not run.

By default, all the compilation outputs (including the executable) are written to the directory of the source file. Pass the --use-root flag to write such outputs to a script-specific location in the scripts directory of the Stack root. The location reflects the absolute path to the source file, but ignoring the drive. This can avoid clutter in the source file directory.

Additional options can be passed to GHC using the --ghc-options option.

Examples

For example, Haskell source file MyScript.hs at location <drive>Users/jane/my-project (where <drive> could be / on Unix-like operating systems or C:/ or similar on Windows):

module Main (main) where

import Data.List (intercalate)
import System.Environment (getArgs)

import Acme.Missiles (launchMissiles)

main :: IO ()
main = do
  advices <- getArgs
  launchMissiles
  putStrLn $ intercalate "\n" advices

can be compiled and run, with arguments, with:

stack --snapshot lts-22.21 script --package acme-missiles --compile MyScript.hs -- "Don't panic!" "Duck and cover!"

All the compilation outputs (like Main.hi, Main.o, and the executable MyScript) will be written to the my-project directory.

If compiled and run with the additional flag --use-root, all the compilation outputs will be written to a directory named MyScript.hs at Users/jane/my-project/ in the scripts directory of the Stack root.

For example, consider the following script extract, based on snapshot Stackage LTS Haskell 20.25, where considerations on Windows differ from non-Windows. The stack script command is specified using Stack's script interpreter.

=== "Windows"

The snapshot includes `Win32` directly. As a consequence, GHC boot packages
`directory`, `process` and `time` (which depend on `Win32`) are all treated
as 'replaced'.

~~~haskell
{- stack script
   --snapshot lts-20.25
   --extra-dep acme-missiles-0.3
   --extra-dep directory-1.3.6.2
   --extra-dep process-1.6.16.0
   --extra-dep time-1.11.1.1
-}

import Acme.Missiles -- from acme-missiles
import Data.Time.Clock.System -- from time
import System.Time.Extra -- from extra

...
~~~

`acme-missiles` is not in the snapshot and so needs to be specified as an
extra-dep.

Stack can deduce that the module imports imply that the required packages
are `acme-missiles`, `time` and `extra` (which is in the snapshot).

`extra` depends on `directory` and `process`. If `directory` and `process`
are not specified as extra-deps, Stack will complain that they have been
'pruned'.

`directory-1.3.6.2` depends on `time < 1.12`. If `time` is not specified as
an extra-dep, Stack will try to construct a build plan based on the latest
version in the package index (which will fail, as the latest version is
`>= 1.12`)

=== "Unix-like"

~~~haskell
{- stack script
   --snapshot lts-20.25
   --extra-dep acme-missiles-0.3
-}

import Acme.Missiles -- from acme-missiles
import Data.Time.Clock.System -- from time
import System.Time.Extra -- from extra

...
~~~

`acme-missiles` is not in the snapshot and so needs to be specified as an
extra-dep.

Stack can deduce that the module imports imply that the required packages
are `acme-missiles`, `time` and `extra` (which is in the snapshot).

All the other dependencies required are either GHC boot packages (which have
not been 'replaced') or in the snapshot.