#1696: Confusing type signature
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Reporter: guest | Owner:
Type: bug | Status: new
Priority: normal | Milestone:
Component: Compiler | Version: 6.6.1
Severity: trivial | Resolution:
Keywords: | Difficulty: Unknown
Os: Unknown | Testcase:
Architecture: Unknown |
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Old description:
> I was working with some buggy numerical code of mine, and I was having
> problems with some types involving exponentiation. My working hypothesis
> was that the problem involved using ^ with a numerical type I had defined
> - I had checked ^'s type through :t and saw:
>
> (^) :: forall a b. (Integral b, Num a) => a -> b -> a
>
> I immediately thought that I needed another type class declaration for my
> new type, and went haring off on that tangent for a long time. Eventually
> someone on #haskell pointed out to me that the *base* could be Num, but
> the power to which it was being raised had to be Integral and that my
> problems stemmed from going foo^(1/3), and that what I needed was more
> along the lines of foo**(1/3).
>
> My confusion stemmed from the variables - the forall declaration goes, in
> order, a-b, and the curried signature itself goes a-b as well, but the
> classes goes b-a! This apparently is for no particular reason, and so I
> think it'd be good if the signatures :t displayed could be a little more
> consistent and go a-b as well, so it'd be instead:
>
> (^) :: forall a b. (Num a, Integral b) => a -> b -> a
>
> A small thing, perhaps, but it did trip me up.
New description:
I was working with some buggy numerical code of mine, and I was having
problems with some types involving exponentiation. My working hypothesis
was that the problem involved using ^ with a numerical type I had defined
- I had checked ^'s type through :t and saw:
{{{
(^) :: forall a b. (Integral b, Num a) => a -> b -> a
}}}
I immediately thought that I needed another type class declaration for my
new type, and went haring off on that tangent for a long time. Eventually
someone on #haskell pointed out to me that the *base* could be Num, but
the power to which it was being raised had to be Integral and that my
problems stemmed from going foo^(1/3), and that what I needed was more
along the lines of foo**(1/3).
My confusion stemmed from the variables - the forall declaration goes, in
order, a-b, and the curried signature itself goes a-b as well, but the
classes goes b-a! This apparently is for no particular reason, and so I
think it'd be good if the signatures :t displayed could be a little more
consistent and go a-b as well, so it'd be instead:
{{{
(^) :: forall a b. (Num a, Integral b) => a -> b -> a
}}}
A small thing, perhaps, but it did trip me up.
Comment (by simonpj):
Was the type of `(^)` declared by you with a type signature, or inferred?
Simon
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/1696>
GHC <http://www.haskell.org/ghc/>
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