问题
other questions and problems, although similar, are not quite like this one. in this specific compiler error, the Haskell GHC won't compile the following code, for the following reason. I don't understand at all - the code is pretty straight forward.
--factorial
fact :: int -> int
fact 0 = 1
fact n | n > 0 = n * fact(n - 1)
main = print (fact 10)
(error:)
No instance for (Ord int) arising from a use of `>'
Possible fix:
add (Ord int) to the context of
the type signature for fact :: int -> int
In the expression: n > 0
In a stmt of a pattern guard for
an equation for `fact':
n > 0
In an equation for `fact': fact n | n > 0 = n * fact (n - 1)
Can you explain the problem to me?
回答1:
Int is what you want:
fact :: int -> int
-->
fact :: Int -> Int
Since in Haskell, types need to begin with a cap.
Edit: Thank Yuras for commenting this:
Or if you want you could use a type class:
fact :: Integral a => a -> a
And you can name the type variable whichever you like, including int. Also, Num might fit your purpose better if you want to define factorial over general numbers.
来源：https://stackoverflow.com/questions/17376071/no-instance-for-ord-int-arising-from-a-use-of-haskell