haskell - Understanding the type of the cast operator in Scrap Your Boilerplate -


in 2003 scrap boilerplate paper laemmel , spj there code snippet on page 3

mkt :: (typeable a, typeable b) => (b -> b) -> -> mkt f = case cast f of g -> g nothing -> id

and paper explains

that is, mkt f x applies f x if x's type same f's argument type

following in pattern of preceding examples in paper, think type of cast f have compared type maybe (b -> b) above evaluate just g, seems incorrect.

what going on here signature of cast f?

the type of castis

cast :: (typeable x, typeable y) => x -> maybe y

it produces nothing if x , y different types , just argument if same. note result type, y must determined calling context when cast used. if isn't, compilation fails unresolved overloading/ambiguous type variable error.

in particular example, types function types, (b -> b) argument, , (a -> a) result. so

cast f :: maybe (a -> a)

and mkt written mkt = frommaybe id . cast.


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