Pattern match in function argument in haskell

Question

In the code defining a function, there is a strange "pattern match" (cradleRootDir -> projdir)

I guess that is meant to apply the function inline and binds the result to the name projdir.

What is the name of that construct ?

withGhcModEnv' :: (IOish m, GmOut m) => (FilePath -> (Cradle -> m a) -> m a) -> FilePath -> Options -> ((GhcModEnv, GhcModLog) -> m a) -> m a
withGhcModEnv' withCradle dir opts f =
    withCradle dir $ \crdl ->
      withCradleRootDir crdl $
        f (GhcModEnv opts crdl, undefined)
 where
   withCradleRootDir (cradleRootDir -> projdir) a = do
       cdir <- liftIO $ getCurrentDirectory
       eq <- liftIO $ pathsEqual projdir cdir
       if not eq
          then throw $ GMEWrongWorkingDirectory projdir cdir
          else a

The constructor

data Cradle = Cradle {
    cradleCurrentDir :: FilePath
  , cradleRootDir    :: FilePath
  , cradleCabalFile  :: Maybe FilePath
  , cradlePkgDbStack  :: [GhcPkgDb]
  } deriving (Eq, Show)

That is a view pattern. See this question for some interesting examples. — duplode
– duplode, Commented Oct 27, 2015 at 11:53

josejuan · Accepted Answer · 2015-10-27 11:56:01Z

7

It's using View Patterns

Evaluation To match a value v against a pattern (expr -> pat), evaluate (expr v) and match the result against pat.

See cabal file

Default-Extensions:   ScopedTypeVariables, RecordWildCards, NamedFieldPuns,
                      ConstraintKinds, FlexibleContexts,
                      DataKinds, KindSignatures, TypeOperators, ViewPatterns
                                                                ^^^^^^^^^^^^
                                                                |  |  |  | |

edited Oct 27, 2015 at 11:56

answered Oct 27, 2015 at 11:50

josejuan

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