Oleg has described a grody hack which achieves this effect. http://okmij.org/ftp/Haskell/types.html#partial-sigs I agree more first class support for this would be nice. Edward Excerpts from Nicholas Tung's message of Thu Jan 19 15:37:28 -0500 2012:
Dear all,
I wanted to voice support for a partial type annotations. Here's my usage scenario: I have a monad for an imperative EDSL, which has an associated expression data type,
class (Monad m, Expression (ExprTyp m)) => MyDSLMonad m where data ExprTyp m :: * -> *
and you write imperative EDSL code like so,
my_code_block = do x <- instruction1 y <- instruction2 (x + x) ...
I want the user to be able to annotate "x is an Integer". However, to do that now, one has to now add a type signature for my_code_block like so, so that the $m$ variable is in scope,
my_code_block :: forall m. MyDSLMonad m => m () my_code_block = do x :: ExprTyp m Integer <- instruction1 ...
If such a feature were available, one could write a nice type synonym "Expr" and use it like so,
type Expr a = ExprTyp _ a
my_code_block = do x :: Expr Integer <- instruction1
Suggestions for workarounds are appreciated. I created an `asExprTypeOf`, similar to Prelude's `asExprTyp`, but I don't like the syntax as much.
Some previous discussion * http://www.haskell.org/pipermail/haskell/2002-April/009409.html * (a reply) http://www.haskell.org/pipermail/haskell/2002-April/009413.html * http://hackage.haskell.org/trac/haskell-prime/wiki/PartialTypeAnnotations
cheers, Nicholas — https://ntung.com — 4432-nstung