The important thing is the type of "floor":
floor :: (Integral b, RealFrac a) => a -> b
That is, if you have a real, fractional type, and you apply floor to
it, you'll get some integral type.
If you look at allocate', you'll see
allocate' :: (RealFrac in, Integral out) => in -> [in] -> [out]
When you apply this function without any other information, you're
applying Haskell's defaulting rules; I believe you'll get Double for
"in" and Integer for "out".
But in allocate, you have two additional declarations:
allocated = sum vs
therefore, allocated :: out
unallocated = n - sum vs
unallocated :: ?
(-) has type (Num a) => a -> a -> a. Since both RealFrac and Integral
have Num as a superclass, that constraint goes away. But you still
end up unifying "in" with "out". Now you need a type that is both a
real-fractional type and an integer type. Unsurprisingly, no such
type exists.
To fix:
unallocated = n - fromIntegral (sum vs)
-- ryan
On Fri, Feb 29, 2008 at 12:09 AM, Lloyd Smith
I have a question about how numeric classes and type checking works. I have two functions that I think should behave the same but don't.
-- I want to split n things up using a given list of fractions -- for example >allocate' 100 [1/3,1/3,1/3] -- [33,33,33] allocate' n fs = vs where vs = map (floor . (*n)) fs
-- I want to find anything left over eventually I will want to -- return what is unallocated as well but for now allocated -- and unallocated are not used! allocate n fs = vs where vs = map (floor . (*n)) fs allocated = sum vs unallocated = n - allocated
When I load these function in the top level everything looks good
[1 of 1] Compiling Main ( allocate.hs, interpreted ) Ok, modules loaded: Main.main *Main> allocate' 100 [1/3,1/3,1/3] [33,33,33] *Main> allocate 100 [1/3,1/3,1/3]
<interactive>:1:0: Ambiguous type variable `t' in the constraints: `Integral t' arising from a use of `allocate' at <interactive>:1:0-25 `RealFrac t' arising from a use of `allocate' at <interactive>:1:0-25 Probable fix: add a type signature that fixes these type variable(s) *Main>
I mixed up my types when finding the allocated and unallocated, but I am not sure why it produces an error when unallocated and allocated are never used? Shouldn't the two functions be compiled down to the same thing?
Suggestions on how to do this more elegantly as well as pointers for understanding numeric type classes would be appreciated.
TIA Lloyd _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe