[Haskell-cafe] partial type annotations

[Edward Z. Yang]

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