Re: [Haskell-cafe] Probably type checker error.

20 Jun 2011


      Yeah, seems to work too.

Отправлено с iPhone

Jun 20, 2011, в 10:55, "Corey O'Connor"  написал(а):
...
Not just a proposal any more. :-) GHC 7.0 does not generalize local let bindings in some situations. See here for information: http://hackage.haskell.org/trac/ghc/blog/LetGeneralisationInGhc7
There is a proposal (from Big Simon) to remove let-generalization: http://research.microsoft.com/en-us/um/people/simonpj/papers/constraints/let...
On 19 Jun 2011, at 18:26, Serguey Zefirov wrote:
...
Right now I write a quite heavy transformation of Haskell source code
and found some strange behaviour of typechecker.
Some prerequisites:
-- dummy class. My own class is much bigger, but I
-- could reproduce that behaviour with that class.
class ToWires a
-- a type with phantom type arguments.
data E ins outs = E
-- a function that relates E and its inputs.
projectInsType :: E ins outs -> ins
projectInsType = error "projectInsType gets evaluated."
-- "register" function.
register :: a -> a -> a
register def a = def
-- a simple addition.
add2 :: (ToWires a, Num a) => (a,a) -> a
add2 (a,b) = a+b
First I have a function:
func :: (ToWires a, Num a) => Maybe a -> a
func mbA = currentSum
      where
              x = case mbA of
                      Just a -> a
                      Nothing -> 0
              nextSum = add2 (x,currentSum)
              currentSum = register 0 nextSum
It typechecks and works just fine after some transformation.
The transformation I work on transform code into something like that:
func_E :: (ToWires a, Num a) => E (Maybe a) a
func_E = r
      where
              r = E
              -- here we relate mbA and r.
              mbA = projectInsType r
              x = case mbA of
                      Just a -> a
                      Nothing -> 0
              nextSum = add2 (x,currentSum)
              currentSum = register 0 nextSum
Note the absence of input of func in transformed func_E. I relate mbA
with it's proper type using binding "mbA = projectInsType r".
Then suddently ghc loses all of the context associated with mbA. And
find type error at the calling of add2.
If I drop ToWires from contexts of func_E and add2 types, all works
fine. If I change add2 to simple addition (x + currentSum), all works
fine.
Full source code is in attachment.
I found it using ghc 6.12.1. I asked colleagues, they told me that the
same error manifests itself in ghc 7.0.3.
Should I fill a bug report or maybe I misunderstood something?
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Re: [Haskell-cafe] Probably type checker error.

MigMit