Haskell: Custom types with conditions

Question

I'm a haskell newbie and I couldn't find an answer to this question.

Can we define types with conditions? For example, simple user-defined data type would be:

data MyList = MyList [a]

Can I somehow modify this code so MyList constructor can take only lists with even number of elements? Something like

data MyList = MyList [a] where (even (length a))

Thank you!

Answer 1

No, you can't.

If it's really necessary, just write a constructor-like function by yourself.

toMyList :: [a] -> MyList
toMyList l | even (length l) = MyList l
           | otherwise       = error "Length of list has to be even"

or if error-checking is likely to occur:

toMyList :: [a] -> Maybe MyList

But depending on the use-case, maybe you can express yourself through types (e.g. tuples or two lists) than through runtime-checks.