Fsharp: Passing N Parameters for a function

user_3462458 0
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I'm trying to create a function that takes in a variable amount of parameters but I can't seem to find any viable solution for F#.

let expression = (fun a b -> a || b)
let expressionTriple = (fun a b c -> (a || b) && c)

// This doesn't work because expression can either be a function that takes fixed arguments
let truthTable numPredicates expression =
    if numPredicates = 2 then
        expression true true
    else
        expression true true false


truthTable 2 expression
truthTable 3 expressionTriple

How can I pass in a variable amount of arguments into the expression function?

f#
2 Answers

In F#, functions with different signatures (including a different number of parameters) are considered distinct types. And any time you want to have a function take a parameter that could be two distinct types (or even a dozen distinct types), you need to use discriminated unions. Here's how you could write your code in a way that will compile and do what you're trying to do:

type Expression<'a> =
    | Double of ('a -> 'a -> 'a)
    | Triple of ('a -> 'a -> 'a -> 'a)

let expression = fun a b -> a || b
let expressionTriple = fun a b c -> (a || b) && c

// This works because expression is a discriminated union
let truthTable expression =
    match expression with
    | Double f -> f true true
    | Triple f -> f true true false

truthTable (Double expression)
truthTable (Triple expressionTriple)

If you wanted to add a four-parameter version, just add a Quad of ('a -> 'a -> 'a -> 'a -> 'a) case to that discriminated union, and so on.

If you have any questions about this, like why I wrote this with generic type 'a instead of bool, please feel free to ask follow-up questions.

user_23145320 
let expression = (fun [a; b] -> a || b)
let expressionTriple = (fun [a; b; c] -> (a || b) && c)

let truthTable numPredicates expression =
   if numPredicates = 2 then
      expression [true; true]
   else
      expression [true; true; false]


truthTable 2 expression 
truthTable 3 expressionTriple
user_89591380
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