Accessing a specific member in a F# tuple

问题

In F# code I have a tuple:

let myWife=("Tijana",32)

I want to access each member of the tuple separately. For instance this what I want to achieve by I can't

Console.WriteLine("My wife is {0} and her age is {1}",myWife[0],myWife[1])

This code doesn't obviously work, by I think you can gather what I want to achieve.

回答1:

You want to prevent your wife from aging by making her age immutable? :)

For a tuple that contains only two members, you can fst and snd to extract the members of the pair.

let wifeName = fst myWife;
let wifeAge = snd myWife;

For longer tuples, you'll have to unpack the tuple into other variables. For instance,

let _, age = myWife;;
let name, age = myWife;;

回答2:

Another quite useful thing is that pattern matching (just like when extracting elements using "let" binding) can be used in other situations, for example when writing a function:

let writePerson1 person =
  let name, age = person
  printfn "name = %s, age = %d" name age

// instead of deconstructing the tuple using 'let', 
// we can do it in the declaration of parameters
let writePerson2 (name, age) = 
  printfn "name = %s, age = %d" name age

// in both cases, the call is the same
writePerson1 ("Joe", 20)
writePerson2 ("Joe", 20)

回答3:

You can use the function fst to get the first element, and snd to get the second ekement. You can also write your own 'third' function:

let third (_, _, c) = c

Read more here: F# Language reference, Tuples

回答4:

You can also write an unpack function for a certain length:

let unpack4 tup4 ind =
    match ind, tup4 with
    | 0, (a,_,_,_) -> a
    | 1, (_,b,_,_) -> b
    | 2, (_,_,c,_) -> c
    | 3, (_,_,_,d) -> d
    | _, _ -> failwith (sprintf "Trying to access item %i of tuple with 4 entries." ind)

let unpack4 tup4 ind =
    let (a, b, c, d) = tup4
    match ind with
    | 0 -> a
    | 1 -> b
    | 2 -> c
    | 3 -> d
    | _ -> failwith (sprintf "Trying to access item %i of tuple with 4 entries." ind)

来源：https://stackoverflow.com/questions/600216/accessing-a-specific-member-in-a-f-tuple

标签

tuples