types from ghc - got them!

Simon Peyton-Jones [email protected]
Tue, 21 Nov 2000 05:48:42 -0800


Well done.

We're blazing away on GHCI (an interactive version of GHC)
at the moment, including
a user interface.  When it's usable, perhaps you'd like to look
at it with a view to adding this feature?

Simon

| -----Original Message-----
| From: Bernard James POPE [mailto:[email protected]]
| Sent: 21 November 2000 06:43
| To: [email protected]
| Cc: [email protected]
| Subject: types from ghc - got them!
| 
| 
| Hi again,
| 
| For my previous example:
| 
|    main = putStr $ show (fred 1 3)
|         where
|         fred a b = let myid x = x in myid (plus a b) 
|         plus x y = x + y 
| 
| I can now get the following output:
| 
|    ==================== Bernie: All Binder TypeSigs 
| ====================
|    main :: IO ()
|    plus :: forall a. (Num a) => a -> a -> a
|    fred :: forall a. (Num a) => a -> a -> a
|    myid :: forall t_aMv. t_aMv -> t_aMv
| 
| Which is exactly what I want. Turned out to be quite simple 
| in the end.
| 
| Thanks a lot for all your help Simon.
| 
| Just out of curiosity, do you think that this sort of output would be
| useful for other people? I don't know what your plans are for 
| GHCi, but you
| may be considering a mode that prints types out, something like :t in
| hugs. I always found it frustrating in hugs that I couldn't 
| get the types
| of locally defined values. My students also find this 
| frustrating, and it
| sometimes causes them to avoid local definitions. Obviously 
| there are issues
| with type variables which are quantified outside the local 
| definition, but
| careful naming of things should be able to fix this.
| 
| Regards,
| Bernie.
| 
| 
| Simon Peyton-Jones writes:
| 
| > Bernie
| > 
| > All (I think) the top level bindings come out of the type check
| > as an AbsBinds.  This is where we generalise from a monomorphic
| > thing to a polymorphic thing.
| > 
| > An AbsBinds has four components, the third of which is a list of
| > triples (TyVar, Id, Id).  You want to grab the first of 
| these Ids (only).
| > You can then ignore the MonoBinds inside the fourth component of
| > the AbsBinds.  So your code will get quite a bit simpler.
| > 
| > The two Ids in the triple are the polymorphic top-level binder and
| > the monomorphic (perhaps recursive) thing that it generalises.
| > It's a bit hard to explain in a short space.  Look at the 
| output of the 
| > desugarer for a simple defn, to see what an AbsBinds translates to.
| > 
| > Anyway these polymorphic Ids will have exactly the for-alls and
| > constraints that you want
| > 
| > hope this gets you moving
| 
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