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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)
535
You do that by putting a name and an @ in front of a pattern. For instance, the pattern xs@(x:y:ys). This pattern will match exactly the same thing as x:y:ys but you can easily get the whole list via xs instead of repeating yourself by typing out x:y:ys in the function body again.
We use pattern matching in Haskell to simplify our codes by identifying specific types of expression. We can also use if-else as an alternative to pattern matching. Pattern matching can also be seen as a kind of dynamic polymorphism where, based on the parameter list, different methods can be executed.
You can only pattern-match on data constructors, and ++ is a function, not a data constructor. Data constructors are persistent; a value like 'c':[] cannot be simplified further, because it is a fundamental value of type [Char] .
Pattern matching is the process of checking whether a specific sequence of characters/tokens/data exists among the given data. Regular programming languages make use of regular expressions (regex) for pattern matching.
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
^^^^^^^^^^^^
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