El mar, 20-07-2010 a las 23:26 -0700, prad escribió:
i'm looking at a factoring function which i've defined as:
factorList = map factors where factors p = [ f | f <- [1..fsq p], p `mod` f == 0 ] fsq :: (RealFrac a, Integral b, Floating a) => a -> b fsq = floor . sqrt
it goes into ghci just fine, but then when i give it something to go i get errors:
Btw, it only loads fine in ghci because you apparently turned of the monomorphism restriction.
*Main> :r Ok, modules loaded: Main. *Main> factorList [4,8,16,32]
<interactive>:1:0: Ambiguous type variable `t' in the constraints: `RealFrac t' arising from a use of `it' at <interactive>:1:0-21 `Integral t' arising from a use of `it' at <interactive>:1:0-21 `Floating t' arising from a use of `it' at <interactive>:1:0-21 Probable fix: add a type signature that fixes these type variable(s)
so i'm not sure where the unhappiness lies because there is no line number.
There is: <interactive>:1:0-21 says the error is in line 1, columns 0-21 of the interactive input. It is saying that factorList is a perfectly fine function for ghci, but it doesn't know what type its argument [4,8,16,32] should be. Should it be a list of integers, floats, doubles, etc? This is because number literals are overloaded in Haskell. And it is suggesting that you tell ghci what type you want that list to be by giving a type signature. The only requirement is that the element type should be in the type-classes 'RealFrac', 'Integral', and 'Floating', which is where the real problem is: there simply is no predefined type that fulfills these type-class constraints. Integral Usually, you don't encounter this problem, because haskell has defaulting rules to resolve exactly this issue. First it tries the type Integer, then the type Double. Here, Integer is in the Integral class, and Double is in the Floating and RealFrac classes, but neither is in all three. And it doesn't make sense for a type to be in both Integral and Floating, they are mutually exclusive. Only ghc does not know this.
i examine fsq more closely, because that looks like the most complicated part of the whole thing and try to define it separately:
*Main> let fsq = floor . sqrt *Main> :t fsq fsq :: (RealFrac a, Integral b, Floating a) => a -> b
This is fine
ok this seems weird to me because i would have thought things are far simpler with
fsq :: Int -> Int
however, i guess because we have a composition here, the compiler wants the fact that
sqrt :: (Floating a) => a -> a floor :: (RealFrac a, Integral b) => a -> b
incorporated in the type?
exactly.
but even if i put the
fsq :: (RealFrac a, Integral b, Floating a) => a -> b
in to the factorList, i get errors,
Well, you just added a type signature that ghc knew already, so what did you expect it to do?
though the fsq works fine on its own.
I guess you mean the following works: *Main> fsq 9 3 However: *Main> fsq (9::Int) <interactive>:1:0: No instances for (Floating Int, RealFrac Int) .... Again, number literals are overloaded, so the 9 without an explicit type signature is defaulted by ghci to a Double, not an Int or an Integer, because ghc knows that Integer doesn't work. If you want fsq to have the type Int -> Int, you have to convert the argument into a floating number first, before you use sqrt. like this: *Main> let fsq = floor . sqrt . fromIntegral *Main> :t fsq fsq :: (Integral b, Integral a) => a -> b But I would suggest to use a proper integer square root function, floating point numbers have limited accuracy :) *Main> let x = 2^53 in fsq (x * x) == x True *Main> let x = 2^53+1 in fsq (x * x) == x False *Main> fsq (9999999999999999^2) 10000000000000000 etc. Jürgen