haskell - GADT type argument not being used for typeclass resolution -

consider following code

data foo f   foo :: foo int  class dynfoo t   dynfoo :: foo f -> foo t  instance dynfoo int   dynfoo foo = foo  obsfoo :: (dynfoo t) => foo f -> foo t obsfoo = dynfoo  usedynfoo :: foo f -> int usedynfoo (obsfoo -> foo) = 1 

the pattern match in usedynfoo should constrain use of obsfoo have type foo f -> foo int, should cause search instance of dynfoo int. however, instead searches instance of dynfoo t unknown t, , naturally fails.

  no instance (dynfoo t0) arising use of ‘obsfoo’     type variable ‘t0’ ambiguous 

however, if change definition of usedynfoo to

usedynfoo :: foo f -> int usedynfoo (obsfoo -> (foo :: foo int)) = 1 

then works, though type signature redundant.

so, why happening, , how can use obsfoo without having give type signature?

it's clearer if write out explicit case (view patterns pretty obscuring wrt type-information flow):

usedynfoo :: foo f -> int usedynfoo foof = case obsfoo foof of     foo -> 1 

here, information f ~ int accessible 1. well, that's not need information: need @ obsfoo foof. , information can't there: gadt pattern matches act complete “type information diode”, i.e. information outside can used within matching scope, no information inside can used without. (for reasons obviously, because information can confirmed @ runtime, when have gadt constructor take from.)

the more interesting question is, why work if add :: foo int signature? well, in particular weirdness of view pattern. see, following not work:

usedynfoo :: foo f -> int usedynfoo foof = case obsfoo foof of     (foo :: foo int) -> 1 

this information, said yourself, redundant.

however turns out view pattern equivalent putting signature on other part of case:

usedynfoo :: foo f -> int usedynfoo foof = case obsfoo foof :: foo int of     foo -> 1 

and quite different pair of shoes, because foo int not inside gadt pattern match.

i don't know why view pattern signatures desugar this, perhaps make pattern possible.