#2187: Top-level bindings are broken for polymorphic values
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Reporter: yallop | Owner:
Type: bug | Status: new
Priority: normal | Milestone: 6.10 branch
Component: Compiler | Version: 6.8.2
Severity: major | Resolution:
Keywords: | Difficulty: Unknown
Testcase: | Architecture: Unknown
Os: Unknown |
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Comment (by yallop):
Thanks for the reply. Yes, I ran into the problem in a real program.
I have code that looks like this:
{{{
(x1,...,xn) = (e1,...en)
where y = e
z = f
}}}
which is generated via Template Haskell. Sometimes the bindings are
supposed to be polymorphic, sometimes not.
It looks like I have two choices, neither of which is particularly
desirable:
1. Change the generation scheme to emit less elegant code, which will
also
complicate the generation code.
2. Require all users of the library to add -XNoMonoPatBinds
From a user's perspective (at least, from this user's), defaulting to
monomorphic pattern bindings looks like quite an unfortunate choice:
1. It doesn't bring any actual benefits to the user (that I can see).
2. It breaks compatibility with Haskell 98, so code that works on other
implementations mysteriously fails without so much as a warning.
3. It generates quite unusable code: there's no realistic use for
functions of type
{{{
GHC.Prim.Any -> GHC.Prim.Any
}}}
so why default to generating that sort of thing?
4. It makes it more difficult to transform programs, since you can no
longer replace
{{{
x = f (a,b)
}}}
with
{{{
(c,d) = (a,b)
f (c,d)
}}}
It's difficult to see how it can simplify the implementation, since you
still have to cope with -XNoMonoPatBinds. There must be *some* reason for
this change, but what can it be?
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2187#comment:2>
GHC <http://www.haskell.org/ghc/>
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