PartialTypeSigs

Example usage (GHC-7.8):

 sigs
     [| ["f1" :: a -> b -> (a, Int),
         "f1" :: b -> a -> (Char, a) ] |]
 
 f1 x y | False = $(unionSigs [| f1 x y |])
 f1 x y = undefined -- (x,y)

A GHC-7.6 compatible version must be slightly longer to work around the extra typechecking done of [| |] brackets:

 sigs [| do
   f2 <- Nothing
   Just [ f2 :: a -> b -> (a, Int),
          f2 :: b -> a -> (Char, a) ]
   |]
   
 f2 x y | False = $(unionSigs [| f2  x y |])
 f2 x y = undefined -- (x,y)

If the expression splice generated by unionSigs is left out,

 sigs
     [| ["g" :: a -> b -> (a, Int),
         "g" :: b -> a -> (Char, a) ] |]
 
 g x y = undefined -- (x,y)

then g's type takes the most general type (g :: t), and the two functions defined by sigs can be used to restrict the type of g:

 partialTypeSig_g1 :: (t -> t1 -> (t, Int)) -> t -> t1 -> (t, Int)
 partialTypeSig_g1 = id
 
 partialTypeSig_g2 :: (t -> t1 -> (Char, t1)) -> t -> t1 -> (Char, t1)
 partialTypeSig_g2 = id

Documentation

sigs :: ExpQ -> DecsQ

any subexpression of the passed-in expression which looks like:

 "functionName" :: t
 functionName :: t

generates the following function:

 partialTypeSig_functionName1 x = x `asTypeOf` (functionName `asTypeOf` (undefined :: t))

Note that the above function is not the same as

 badId x = x `asTypeOf` (functionName :: t)

which requires that t be more specific than functionName

unionSigs :: ExpQ -> ExpQ