Question

i have something like

type A = 
   | X of string
   | Y of int

i have a sequence of X types, [X "foo"; X "boo"; X "moo"]
is there a shortcut for doing a map to convert it to ["foo"; "boo"; "moo"] without doing a match?

Thanks!

Answer 1

I wouldn’t generally use a solution that doesn’t handle all cases of the pattern matching (e.g. when using fun (X str) -> .... It is always a good idea to add handler, even if it just reported a more informative error (such as, that the case was unexpected because it was filtered before).

You can extend kvb’s solution using the function syntax (which is like fun with pattern matching):

List.map (function (X s) -> s | _ -> failwith "Unexpected case")

Alternatively, if you wanted to ignore Y values (so that [X "a"; Y 1; X "b"] becomes ["a"; "b"]) you can use List.choose function:

List.choose (function (X s) -> Some s | _ -> None)

To write this using list comprehensions, you’d need to use full-blown pattern matching using match, so it would be a bit longer than using higher-order functions.