typeclass - Haskell Recursive Type Classes -

i want create recursive instance type based on tuples. looking similar this:

class provider b   getinstance :: -> b  instance provider b => provider (x, a) b   getinstance (x, a) = getinstance  instance provider (b, x) b   getinstance (b, _) = b  tryfunc1 :: int tryfunc1 =   let provider = ("test", (10, ())) :: (string, (int, ()))   in getinstance provider  tryfunc2 :: string tryfunc2 =   let provider = ("test", (10, ())) :: (string, (int, ()))   in getinstance provider 

unfortunatelly, haskell fails solve instance. reason?

the solution stop using deprecated overlappinginstances pragma , start using per instance overlapping , overlappable pragmas. change:

instance {-# overlappable #-} provider b => provider (x, a) b   getinstance (x, a) = getinstance  instance {-# overlapping #-} provider (b, x) b   getinstance (b, _) = b 

i tryfunc1 10 , tryfunc2 "test".

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technically need either overlappable or overlapping pragma, believe practice have both in case... also, suppose behaviour want, note gets first of whatever type looking (so getinstance (10, (20, ())) :: int gives me 10 , not 20)

good source of info ticket tracking feature's creation.