#7295: bad code for Double literals
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Reporter: jwlato | Owner: igloo
Type: bug | Status: new
Priority: normal | Milestone:
Component: Compiler | Version: 7.6.1
Keywords: | Os: Unknown/Multiple
Architecture: Unknown/Multiple | Failure: None/Unknown
Difficulty: Unknown | Testcase:
Blockedby: | Blocking:
Related: |
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Changes (by simonpj):
* owner: => igloo
* difficulty: => Unknown
Comment:
Good point. Here's what is happening.
* In `map (* (-0.5)) xs :: [Double]`, the typechecker and/or desugarer
can see a totally mono-typed use of the literal `0.5` at `Double` and so
genreates exactly that literal.
* But in your main example, you define a polymorphic function `f`, whose
type is
{{{
f :: Fractional a => a -> a
}}}
Sure, `f` has only one call site but the typechecker/desugarer don't know
that, so they generate `fromRational d (negate d (1 % 2))`, where `d` is
the fractional dictionary.
So, good as the typechecker/desugarer short-cut is for literals, your
example points out that we should do better constant folding for literals
that start life overloaded, but become specialised. This will also happen
when we have a top-level overloaded function that gets specialised.
My solution. In `GHC.Float` we see
{{{
instance Fractional Float where
fromRational = blah blah blah
}}}
Instead we want
{{{
instance Fractioal Float where
{-# INLINE fromRational #-}
fromRational (a :% b) = rationalToFloat a b
rationalToFloat :: Integer -> Integer -> Float
{-# NOINLINE [1] rationalToFloat
rationalToFloat = blah blah blah -- As before
}}}
AND we want a constant-folding rule in `PrelRules` for `rationalToFloat`
that spots two `Integer`-literal arguments and produces the right float;
just like `TcHsSyn.shortCutLit` does.
Same for `fromRational` at `Double`.
We already have similar mechanisms for `fromInteger`, so I think that's
all right.
Simon
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7295#comment:1>
GHC <http://www.haskell.org/ghc/>
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