F# - Getting fst element of tuple from option type function

user_3996969 0
Viewed 158

I am trying to access the fst element of a tuple. Normally I use fst (tuple), but this situation is a little different.

let getCard (pl : player) : (card * player) option =
    let plDeck = pl 
    match plDeck with
    | c1::re -> Some (c1,(re)) 
    | [] -> None

This is my f# code. the player type is a list of ints, and the output is tuple with the first int of the player list, and the player list minus the first int.

It's an assignment from my computer science class, so it is required that I use the option type.

I am trying to access the fst element of the tuple in another function by writing

let gc = fst (getCard [1,2,3])

but it seems like I can't do it this way, since I am getting the warning:

This expression was expected to have type ''a * 'b' but here has type '(card * player) option'

How do I work around this?

option-type tuples f#
1 Answers

The compiler is telling you that you're trying to access an option of tuple card * player while the function fst expects a tuple of card * player.

You could pattern match on your getCard function and extract the card.

let result = 
    match getCard [1..5] with
    | Some card -> fst(card)
    | None -> -1

You could also use pattern matching to extract the first part of your tuple.

let result = 
    match getCard [1..5] with
    | Some (card, _) -> card
    | None -> -1

As suggested by @Guran you shouldn't return magic numbers

let result = 
    match getCard [1..5] with
    | Some (card, _) -> Some card
    | None -> None
user_134082970
Related