Re: [Haskell] question about a failure to generalize
> If so, try working around the monomorphism restriction by changing > from a pattern binding to a function binding. > > fold f = foldRegsUsed f Brilliant. I hadn't known about the monomorphism restriction. Now I know it's a 'necessary evil'. Thanks! Norman ___ Haskell mailing list Haskell@haskell.org http://www.haskell.org/mailman/listinfo/haskell
Re: [Haskell] question about a failure to generalize
Quoting Stefan O'Rear <[EMAIL PROTECTED]>: On Mon, Sep 17, 2007 at 04:15:10PM +1200, Tom Pledger wrote: Norman Ramsey wrote: : | This code fails to compile because the compiler is willing to | use 'fold' at only one type (CmmExpr as it happens) : When it failed to compile, was fold = foldRegsUsed a top-level declaration in the module, rather than local to foldRegsUsed? If so, try working around the monomorphism restriction by changing from a pattern binding to a function binding. fold f = foldRegsUsed f The monomorphism restriction is not affected by top-level-or-not, see sections 4.5.1 and 4.5.5 in the Haskell 98 Language and Libraries Report. Stefan Aargh! Sorry, I'm having a Bad Details Day. 3 in 1 message: - Overlooking the "to make the code work, I had to expand 'fold' into 'foldRegsUsed' everywhere it appears" part in the original question, - Chopping a letter out of Norman's email address, and - Mixing up the MR with other issues, possibly something from the binding groups section of Typing Haskell In Haskell. The one saving grace is that I tested my suggestion before posting, and it worked. :-) - Tom ___ Haskell mailing list Haskell@haskell.org http://www.haskell.org/mailman/listinfo/haskell
Re: [Haskell] question about a failure to generalize
On Mon, Sep 17, 2007 at 04:15:10PM +1200, Tom Pledger wrote: > Norman Ramsey wrote: > : > | This code fails to compile because the compiler is willing to > | use 'fold' at only one type (CmmExpr as it happens) > : > > When it failed to compile, was > > fold = foldRegsUsed > > a top-level declaration in the module, rather than local to foldRegsUsed? > > If so, try working around the monomorphism restriction by changing from a > pattern binding to a function binding. > > fold f = foldRegsUsed f The monomorphism restriction is not affected by top-level-or-not, see sections 4.5.1 and 4.5.5 in the Haskell 98 Language and Libraries Report. Stefan signature.asc Description: Digital signature ___ Haskell mailing list Haskell@haskell.org http://www.haskell.org/mailman/listinfo/haskell
[Haskell] question about a failure to generalize
Norman Ramsey wrote: : | This code fails to compile because the compiler is willing to | use 'fold' at only one type (CmmExpr as it happens) : When it failed to compile, was fold = foldRegsUsed a top-level declaration in the module, rather than local to foldRegsUsed? If so, try working around the monomorphism restriction by changing from a pattern binding to a function binding. fold f = foldRegsUsed f Regards, Tom ___ Haskell mailing list Haskell@haskell.org http://www.haskell.org/mailman/listinfo/haskell
[Haskell] question about a failure to generalize
Dear Haskellers,
I've had a Haskell program rejected for reasons I don't understand.
Here's the relevant bit of code; the problem is that I expected the
type-inference engine to generalize the abbreviation 'fold' to an
overloaded function, but it doesn't---to make the code work, I had to
expand 'fold' into 'foldRegsUsed' everywhere it appears. I'm baffled
because 'fold' isn't mutually recursive with anything, so I would have
expected ordinary Hindley-Milner style inference to generalize it to
something of the type
UserOfLocalRegs a => (b -> LocalReg -> b) -> b -> a -> b
But that's not what happens. This code fails to compile because the
compiler is willing to use 'fold' at only one type (CmmExpr as it happens):
class UserOfLocalRegs a where
foldRegsUsed :: (b -> LocalReg -> b) -> b -> a -> b
instance UserOfLocalRegs Middle where
foldRegsUsed f z m = middle m
where middle (MidComment {})= z
middle (MidAssign _lhs expr) = fold f z expr
middle (MidStore addr rval) = fold f (fold f z addr)
rval
middle (MidUnsafeCall tgt _ress args) = fold f (fold f z tgt) args
middle (CopyIn _ _formals _) = z
middle (CopyOut _ actuals)= fold f z actuals
fold = foldRegsUsed
What rule of the language have I overlooked?
Norman
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