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f#generic-collections

Specifying a generic parameter in a double list in F#

I am creating a function in F# which is given a list of lists, and which returns true, if all lists are of the same length, except if this length is 0.

I want to specify that the entries of the inner list can be a generic type 'T, so I was going for

let properTable (input : list list<'T>) : bool =
   let lengths = set ([for i in 0..(input.Length-1) -> input.[i].Length])
   not (lengths.Contains(0)) && lengths.Count = 1

When pasting this function into F# interactive, I get the error message:

stdin(148,35): error FS0010: Unexpected type application in pattern. Expected ')' or other token.

If I replace the type the type of input with int list list, the function works, but I'd like any type, not just an int.

I can get it to work with only a single list, as in

let testSingleList (input : list<'T>) : bool =
    input.Length > 0

How can I accomplish a similar construction, but for a list of lists?

Solution

  • You're mixing two different "styles" of type annotation.

    Using either

    'T list list

    or

    List<List<'T>>

    should work.