|Version 1 (modified by augustss, 4 years ago) (diff)|
Most language entities in Haskell can be named so that they can be abbreviated instead of written out in full. This proposal provides the same power for patterns.
Here is a simple representation of types
data Type = App String [Type]
Using this representations the arrow type looks like App "->" [t1, t2]. Here is a function to collect are all argument types of nested arrow:
collectArgs :: Type -> [Type] collectArgs (App "->" [t1, t2]) = t1 : collectArgs t2 collectArgs _ =  isInt (App "Int" ) = True isInt _ = False
Matching on the arrow type is both hard to read and error prone to write.
The proposal is to introduce a way to give pattern names:
pattern Arrow t1 t2 = App "->" [t1, t2] pattern Int = App "Int" 
And now we can write
collectArgs :: Type -> [Type] collectArgs (Arrow t1 t2) = t1 : collectArgs t2 collectArgs _ =  isInt Int = True isInt _ = False
Furthermore, the pattern synonym can also be used in expressions, e.g.,
arrows :: [Type] -> Type -> Type arrows = foldr Arrow