Ambiguous instance in Haskell

Question

I just write a simple code in Haskell,

coo x y = ((lim-1)*y*(y-1)`div`2) + (y*(y-1)*(sum (map (\j->(x`div`j)-j)  [2..lim] )))
  where
    lim = floor (sqrt x)

but when I use 'coo 10 10' in ghci, it gives me the following error:

<interactive>:3:1:
No instance for (Floating a0) arising from a use of ‘it’
The type variable ‘a0’ is ambiguous
Note: there are several potential instances:
  instance Floating Double -- Defined in ‘GHC.Float’
  instance Floating Float -- Defined in ‘GHC.Float’
In the first argument of ‘print’, namely ‘it’
In a stmt of an interactive GHCi command: print it

What happened? I am kind of sure I match all types correct.

You're going to need an explicit type signature for (or within) coo 10 10 itself. To see why, type :t coo in GHCi. The types are correct, but coo 10 10 is still polymorphic. You'll have to specify whether you want coo 10 10 :: Float or coo 10 10 :: Double. — user824425
– user824425, Commented Apr 1, 2016 at 8:10
@Rhymoid nope ... although the error would indicate this it is not the problem here - try it: you will just get another error (telling you Float or Double is not Integral) — Random Dev
– Random Dev, Commented Apr 1, 2016 at 8:10
sqrt :: Floating a => a -> a; you can use sqrt (fromIntegral x), but be careful of precision loss. — Ry-
– Ry- ♦, Commented Apr 1, 2016 at 8:11
@Carsten I agree it's not a complete answer, but it is an answer for the immediate problem. Learning how to debug is vital in learning a language. — user824425
– user824425, Commented Apr 1, 2016 at 8:14

Random Dev · Accepted Answer · 2016-04-01 08:09:07Z

4

If you look closely (or ask GHCi) you will see that your function has the type

coo :: (Floating a, Integral a, RealFrac a) => a -> a -> a

telling GHC to use some type a that is both a Floating and a Integral and this will get difficult (there are no such types in the Prelude)

I am not 100% sure what you are trying to do but one way to fix this is to change lim into:

lim = floor (sqrt $ fromIntegral x)

this will yield

λ> coo 10 10
360

edited Apr 1, 2016 at 8:09

answered Apr 1, 2016 at 8:06

Random Dev

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4 Comments

CYC Over a year ago

Could you suggest how to fix this problem?

Random Dev Over a year ago

done - but I don't know what your function is supposed to be doing

user824425 Over a year ago

The proper way would be to implement the integer square root function.

Random Dev Over a year ago

@Rhymoid sure but this was not the question here

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