Type system extensions in GHC
GHC comes with a rather large collection of type-system extensions (beyond Haskell 98). They are all documented in the user manual, but this page is a place to record observations, notes, and suggestions on them.
Type signatures and ambiguity
It's quite common for people to write a function definition without a type signature, load it into GHCi, use :t to see what type it has, and then cut-and-paste that type into the source code as a type signature. Usually this works fine, but alas not always. Perhaps this is a deficiency in GHC, but here's one way it can happen:
class C a b where foo :: a -> b konst :: a -> Bool konst x = True f :: (C a b) => a -> Bool f x = konst (foo x)
If you comment out the type signature
f :: (C a b) => a -> Bool, the module will load fine into GHCi, and :t will report exactly this type for f. But if you leave the type signature in, you'll get this error:
Foo1.hs:12:13: Could not deduce (C a b1) from the context (C a b) arising from use of `foo' at Foo1.hs:12:13-17 Possible fix: add (C a b1) to the type signature(s) for `f' In the first argument of `konst', namely `(foo x)' In the expression: konst (foo x) In the definition of `f': f x = konst (foo x)
What's going on? Without the type signature, GHC picks a type for x, say x::a. Then applying foo means GHC must pick a return type for foo, say b, and generates the type constraint (C a b). The function konst just discards its argument, so nothing further is known abouut b. So GHC ends up saying that
f :: (C a b) => a -> Bool.
This is probably a very stupid type. Suppose you called f thus: (f 'a'). Then you'd get a constraint (C Char b) where nothing is known about b. That would be OK if there was an instance like:
instance C Char b where ...
Here an interesting message about the interaction of existential types and overlapping instances.
Indexed data types and indexed newtypes
Indexed data types (including associated data types) are a very recent addition to GHC's type system extensions that is not yet included in the user manual. To use the extension, you need to obtain a version of GHC from its source repository.
Stand-alone deriving clauses
Bjorn Bringert has recently implemented "stand-alone deriving" declarations.