Haskell - 签名和类型错误

Question

I'm new to Haskell and I'm having some trouble with function signature and types. Here's my problem:

I'm trying to make a list with every number between 1 and 999 that can be divided by every numeral of it's own number. For example the number 280 can be in that list because 2+8+0=10 and 280/10 = 28 ... On the other hand 123 can't because 1+2+3=6 and 123/6=20,5. When the final operation gives you a number with decimal it will never be in that list.

Here's my code:

let inaHelper x = (floor(x)`mod`10)+ (floor(x/10)`mod`10)+(floor(x/100)`mod`10)

This first part will only do the sum of every numeral of a number. And this part works...

Here's the final part:

let ina = [x | x <- [1..999] , x `mod` (inaHelper x) == 0   ]

This final part should do the list and the verification if it could be on the list or not. But it's give this error:

No instance for (Integral t0) arising from a use of ‘it’
    The type variable ‘t0’ is ambiguous
    Note: there are several potential instances:
      instance Integral Integer -- Defined in ‘GHC.Real’
      instance Integral Int -- Defined in ‘GHC.Real’
      instance Integral Word -- Defined in ‘GHC.Real’
    In the first argument of ‘print’, namely ‘it’
    In a stmt of an interactive GHCi command: print it


...

Answer 1

ina = [x | x <- [1..999] , x `mod` (inaHelper x) == 0   ]

What is the type of x ? Integer ? Int ? Word ? The code above is very generic, and will work on any integral type. If we try to print its type we get something like this

> :t ina
ina :: (Integral t, ...) => [t]

meaning that the result is a list of any type t we want, provided t is an integral type (and a few other constraints).

When we ask GHCi to print the result, GHCi needs to choose the type of x , but can not decide unambiguously. This is what the error message states.

Try specifying a type when you print the result. Eg

> ina :: [Int]

This will make GHCi choose the type t to be Int , removing the ambiguity.