[Haskell-cafe] Strange type behavior in GHCi 6.4.2

[Kirsten Chevalier]

On 12/28/06, Grady Lemoine <grady.lemoine at gmail.com> wrote:
>
> One question I have, though, for anyone on the list who knows the
> answer -- if I give a function a polymorphic type signature, can it
> affect performance?  That is, if I write two functions with the same
> definitions but different user-specified type signatures,
>
> foo1 :: (Floating a) => a -> a
> foo1 = ...
>
> and
>
> foo2 :: Double -> Double
> foo2 = ...
>
> when I apply foo1 to a Double, will the compiler (GHC, specifically)
> generate code that is just as efficient as if I used foo2?
>

As with so many things, the answer is "it depends". The simple answer
to "if I give a function a polymorphic type signature, can it affect
performance?" is "no, because type information is erased before code
generation". However, if by "polymorphic type signatures" you mean
ones involving class-based overloading, like the ones you wrote, then
the answer is "maybe" -- functions that have classes involved in their
types are desugared by the compiler into functions that take extra
"dictionary" arguments, which can make performance worse. GHC does
"specialization" to try to negate some of the performance impact of
this, but it doesn't always do what you want it to. The best thing to
do is to make sure you compile your code with -O2 and if profiling
seems to imply that overloaded functions are causing you bottlenecks,
seek help on this list or on Haskell IRC (and there should be a pretty
good body of previous discussion on the subject in the list archives).

Cheers,
Kirsten

-- 
Kirsten Chevalier* chevalier at alum.wellesley.edu *Often in error, never in doubt
"I eat too much / I laugh too long / I like too much of you when I'm
gone." -- Ani DiFranco