Strange type-related error
- by vsb
I wrote following program:
isPrime x = and [x `mod` i /= 0 | i <- [2 .. truncate (sqrt x)]]
primes = filter isPrime [1 .. ]
it should construct list of prime numbers. But I got this error:
[1 of 1] Compiling Main ( 7/main.hs, interpreted )
7/main.hs:3:16:
Ambiguous type variable `a' in the constraints:
`Floating a' arising from a use of `isPrime' at 7/main.hs:3:16-22
`RealFrac a' arising from a use of `isPrime' at 7/main.hs:3:16-22
`Integral a' arising from a use of `isPrime' at 7/main.hs:3:16-22
Possible cause: the monomorphism restriction applied to the following:
primes :: [a] (bound at 7/main.hs:3:0)
Probable fix: give these definition(s) an explicit type signature
or use -XNoMonomorphismRestriction
Failed, modules loaded: none.
If I specify signature for isPrime function explicitly:
isPrime :: Integer -> Bool
isPrime x = and [x `mod` i /= 0 | i <- [2 .. truncate (sqrt x)]]
I can't even compile isPrime function:
[1 of 1] Compiling Main ( 7/main.hs, interpreted )
7/main.hs:2:45:
No instance for (RealFrac Integer)
arising from a use of `truncate' at 7/main.hs:2:45-61
Possible fix: add an instance declaration for (RealFrac Integer)
In the expression: truncate (sqrt x)
In the expression: [2 .. truncate (sqrt x)]
In a stmt of a list comprehension: i <- [2 .. truncate (sqrt x)]
7/main.hs:2:55:
No instance for (Floating Integer)
arising from a use of `sqrt' at 7/main.hs:2:55-60
Possible fix: add an instance declaration for (Floating Integer)
In the first argument of `truncate', namely `(sqrt x)'
In the expression: truncate (sqrt x)
In the expression: [2 .. truncate (sqrt x)]
Failed, modules loaded: none.
Can you help me understand, why am I getting these errors?
