Extend FunD data constructor with information about type signature

I want to propose that FunD data constructor in Template Haskell is extended with a field that contains that function's type signature (if present). Currently the type signature is a separate declaration represented with SigD. I motivate my proposal with the fact that function and its type signature are not independent and it might be the case that processing one requires knowledge of the other.

Here's a concrete example. Recently I've been working together with Richard Eisenberg on promoting higher-order functions to the type level. To promote a higher order function I need to replace applications of a function passed as a parameter with usage of a special Apply type family. For example the definition of map:

map :: (a -> b) -> [a] -> [b]
map _ [] = []
map f (x:xs) = f x : map f xs

will be promoted to:

type family Map (f :: (TyFun k1 k2) -> *) (list :: [k1]) :: [k2] where
  Map f '[] = '[]
  Map f (x ': xs) = Apply f x ': Map f xs

To generate equations in the Map type family from declarations stored by a FunD data constructor I need to know that f is a function. The only way for me to know this is by promoting type signature stored in SigD and checking the type of f. But to promote type signature to a type family I need to know the number of patterns in a function clause, which again is stored in FunD. In other words my algorithm looks like this:

1. Recover number of patterns in a function clause
2. Based on number of paterns in function's clause promote the type signature to a type family
3. Based on promoted type signature and definitions in FunD generate equations in the type family.

Since FunD and SigD are independent I need to do three separate passes (or two passes + some hacking). If FunD contained informatuon about type signature I could achieve the same in one pass.

Trac metadata

Trac field	Value
Version	7.7
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Template Haskell
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