Safe and polymorphic toEnum
- by jetxee
I'd like to write a safe version of toEnum:
safeToEnum :: (Enum t, Bounded t) => Int -> Maybe t
A naive implementation:
safeToEnum :: (Enum t, Bounded t) => Int -> Maybe t
safeToEnum i =
if (i >= fromEnum (minBound :: t)) && (i <= fromEnum (maxBound :: t))
then Just . toEnum $ i
else Nothing
main = do
print $ (safeToEnum 1 :: Maybe Bool)
print $ (safeToEnum 2 :: Maybe Bool)
And it doesn't work:
safeToEnum.hs:3:21:
Could not deduce (Bounded t1) from the context ()
arising from a use of `minBound' at safeToEnum.hs:3:21-28
Possible fix:
add (Bounded t1) to the context of an expression type signature
In the first argument of `fromEnum', namely `(minBound :: t)'
In the second argument of `(>=)', namely `fromEnum (minBound :: t)'
In the first argument of `(&&)', namely
`(i >= fromEnum (minBound :: t))'
safeToEnum.hs:3:56:
Could not deduce (Bounded t1) from the context ()
arising from a use of `maxBound' at safeToEnum.hs:3:56-63
Possible fix:
add (Bounded t1) to the context of an expression type signature
In the first argument of `fromEnum', namely `(maxBound :: t)'
In the second argument of `(<=)', namely `fromEnum (maxBound :: t)'
In the second argument of `(&&)', namely
`(i <= fromEnum (maxBound :: t))'
As well as I understand the message, the compiler does not recognize that minBound and maxBound should produce exactly the same type as in the result type of safeToEnum inspite of the explicit type declaration (:: t). Any idea how to fix it?
