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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.

Global configuration files

Non-project level configuration options in global configuration files (config.yaml) are not ignored by the stack script command.

!!! info

Non-project level configuration options may be useful if
[`allow-newer`](../configure/yaml/non-project.md#allow-newer) and/or
[`allow-newer-deps`](../configure/yaml/non-project.md#allow-newer-deps) are
required.

Project-level configuration file

The stack script command ignores any project-level configuration file (stack.yaml, by default), including in the global-project directory in the Stack root.

!!! info

The `stack script` command can be contrasted with the
[`stack ghc`](ghc_command.md) and [`stack runghc`](runghc_command.md)
commands, which do not ignore any project-level configuration file.

GHC

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

Snapshot and extra-deps

A snapshot must be specified on the command line, using the --snapshot option. For example:

stack script --snapshot lts-22.28 MyScript.hs

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

GHC boot packages that have been 'replaced' (see further below) can be specified as an --extra-dep.

Required packages

The names of required packages can be either deduced or specified.

The base package associated with the version of GHC specified by the snapshot is always available.

If no packages are specified, 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. In that regard, Stack assumes that:

  • a line that begins import is an import statement;
  • import may be followed by qualified on the same line;
  • consistent with GHC's PackageImports language extension, that if import or import qualified is followed by "<name>" on the same line, that <name> is the name of a required package;
  • otherwise, import or import qualified is followed by the module name on the same line. Stack will not deduce the names of hidden packages from module names or the names of blacklisted packages.

!!! note

The first time that Stack deduces package names from module names can take
some time. Use the `--verbose` option to understand Stack's progress.

!!! info

Certain packages are blacklisted because they expose one or more modules
with names that are the same as modules exposed by more popular packages.
The blacklisted packages are `Glob`, `HTF`, `async-dejafu`,
`binary-ieee754`, `cipher-aes`, `cipher-blowfish`, `cipher-camellia`,
`cipher-des`, `cipher-rc4`, `control-monad-free`, `courier`, `crypto-api`,
`crypto-cipher-types`, `crypto-numbers`, `crypto-pubkey`, `crypto-random`,
`cryptohash`, `cryptohash-conduit`, `cryptohash-md5`, `cryptohash-sha1`,
`cryptohash-sha256`, `fay-base`, `gl`, `gtk3`, `hashmap`, `hledger-web`,
`hxt-unicode`, `kawhi`, `language-c`, `log`, `monad-extras`, `monads-tf`,
`nanospec`, `newtype-generics`, `objective`, `plot-gtk3`, `prompt`,
`regex-compat-tdfa`, `regex-pcre-builtin`, `rerebase`, `svg-tree` and `zip`.

Alternatively, 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 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 is 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.

!!! warning

GHC has the concept of 'installed packages' (which differ from 'Cabal
packages') in package databases. An installed package has a name. An
installed package corresponding to the main (unnamed) library of a Cabal
package has the same name as the Cabal package. An installed package
corresponding to a sub-library of a Cabal package has a 'munged' name that
reflects the name of the Cabal package and the name of the sub-library. An
installed package corresponding to a sub-library also has a `package-name`,
which is the name of the Cabal package.

The `--package` option of `stack script` makes use of GHC's `-package`
option to expose an installed package. Unfortunately, the latter treats
`package-name` (if it exists) as if it were also the name of the installed
package. That means, for a Cabal package with one or more sub-libraries, the
GHC option `-package=<name>` cannot distinguish between (a) the installed
package `<name>` corresponding to the main library of Cabal package `<name>`
and (b) an installed package corresponding to a sub-library of that Cabal
package. The installed package that GHC picks to expose is indeterminate.
This can cause GHC to pick the wrong installed package and to report that it
cannot load a module because it is a member of a hidden package.

Compilation

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.

GHC options

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

Script arguments

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

Examples

Example 1

A 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-22.28 script --package acme-missiles --compile MyScript.hs -- "Don't panic!" "Duck and cover!"

acme-missiles-0.3 (the most recent version in the package index) will be used.

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.

Example 2

As for Example 1, but acme-missiles-0.2 is specified by adding it to the snapshot as an extra-dep. The stack script command is specified using Stack's script interpreter.

{- stack script
   -- snapshot lts-22.28
   -- extra-dep acme-missiles-0.2
   -- package acme-missiles
-}
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
stack MyScript.hs "Don't panic!" "Duck and cover!"

Example 3

Stackage snapshot LTS Haskell 20.25 includes GHC boot package Win32 directly. On Windows only, GHC boot packages Cabal, directory, process and time all depend on Win32 and, consequently, are all treated as 'replaced'. Consequently, for example, Stack will:

  • on Windows, try to construct a build plan based on the latest version of Cabal in the package index; and
  • on non-Windows, use the boot package in the build plan (because Cabal is not 'replaced').

Consider also 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"

~~~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.

Example 4

A Haskell source file MyScript.hs, as follows:

{- stack script
   --snapshot lts-22.28
-}

{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE PackageImports    #-}

module Main (main) where

import "text" Data.Text (Text (..), unpack)

main :: IO ()
main = putStrLn $ unpack "This is text."

As module Data.Text is exposed by a number of packages that are included, directly or indirectly, in the specified snapshot (incipit-base, incipit-core, relude and text), PackageImports and "text" are required to specify which module is being imported.