auto-split
A GHC plugin that performs automatic case splitting.
Usage
This plugin is intended to be used with GHCi or adjacent utilities such as
ghcid and ghciwatch as a developement tool, not as a package dependency.
Here is an example command for starting a REPL for a stack project with the
auto-split plugin enabled:
stack repl my-project --package auto-split --ghci-options='-fplugin AutoSplit'
likewise for a cabal project:
cabal repl my-project --build-depends auto-split --repl-options='-fplugin AutoSplit'
With the plugin enabled, use SPLIT in a pattern match for a case statement or
function declaration. Compiling the module will then result in that pattern
match being expanded to cover all missing cases.
For example, compiling this module
module Foo where
foo :: Maybe Bool -> String
foo x = case x of
SPLIT -> undefined
results in the module file being updated to
module Foo where
foo :: Maybe Bool -> String
foo x = case x of
Just _ -> undefined
Nothing -> undefined
if the module is then changed to
module Foo where
foo :: Maybe Bool -> String
foo x = case x of
Just SPLIT -> "foo"
Nothing -> "bar"
compiling will result in
module Foo where
foo :: Maybe Bool -> String
foo x = case x of
Just True -> "foo"
Just False -> "foo"
Nothing -> "bar"
The SPLIT pattern can be used in this way anywhere a pattern match group
occurs: case statements, lamba case, or function declarations.
If case splitting results in constructors or patterns that are not in scope,
they will be qualified with NOT_IN_SCOPE.
Compilation will abort when case splitting occurs since the module file has
been updated. This is done by having GHC emit a custom error. This error does
not indicate that something went wrong.
Known limitations
- A module must pass the type checker for splitting to occur, unless the
-fdefer-type-errors GHC flag is used.
- Using SPLIT in pattern match will insert patterns for all missing cases in
the group. It doesn't restrict to the position where SPLIT is used.
- Doesn't work well with the view patterns syntax extension
- Doesn't apply to code inside CPP conditional blocks
- The plugin only supports certain GHC versions with the intent of supporting
the four latest release series, i.e.
9.6.* thru 9.12.*. The cabal file
indicates the currently supported versions.
