Haskell：不在范围内：数据构造函数

Question

Started to learn haskell today for school and I run into a problem with function. I don't understand why it's not in the scope..

Heres the code:

ff :: [[Char]] -> [[Char]] -> [Char]
ff A B = [[x !! 0, y !! 1] | x <- A, y <- B, (x !! 1) == (y !! 0)]

And errors:

md31.hs:2:4: Not in scope: data constructor `A'

md31.hs:2:6: Not in scope: data constructor `B'

md31.hs:2:38: Not in scope: data constructor `A'

md31.hs:2:46: Not in scope: data constructor `B'

Thanks in advance :)

Answer 1

Function parameters have to start with a lowercase letter in Haskell.

As such, you'd need to make A and B lowercase ( a and b ) in your function definition.

If the first letter of an identifier is in uppercase, it is assumed to be a data constructor .

Answer 2

In Haskell the capital letters mean that value is data constructor as in:

data Test = A | B

If you need variable use lowercase:

ff a b = [[x !! 0, y !! 1] | x <- a, y <- b, (x !! 1) == (y !! 0)]