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The Haskell const function

The const function is simple, but you can use it to make your code more legible. In this example we convert a unary function to a function of arity 2 (that ignores the second argument). Also by using the flip function from Haskell (which is equally easy to define) you can ignore the first argument.

Declarations

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// The const function
let ct a _ = a

// The flip function (not used in this snippet)
let flip f x y = f y x

Use case

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// Code taken from a toy interpreter

type Exp = 
    // ...
    // Lambdas are expressed as F# an function that takes a list of Exps and an env
    | Lambda of (Exp list -> Env -> Exp)
    // ...
and Env = (string * Exp) list

// Built-ins receive evaluated arguments so they don't care about the env.
// By composing the function we get with ct we're basically converting a function
// of arity 1 to a function of arity 2 that just ignores its second argument

let builtin f = Lambda (f >> ct)

val ct : a:'a -> 'b -> 'a

Full name: Script.ct

val a : 'a

val flip : f:('a -> 'b -> 'c) -> x:'b -> y:'a -> 'c

Full name: Script.flip

val f : ('a -> 'b -> 'c)

val x : 'b

val y : 'a

type Exp = | Lambda of (Exp list -> Env -> Exp)

Full name: Script.Exp

union case Exp.Lambda: (Exp list -> Env -> Exp) -> Exp

type 'T list = List<'T>

Full name: Microsoft.FSharp.Collections.list<_>

type Env = (string * Exp) list

Full name: Script.Env

Multiple items
val string : value:'T -> string

Full name: Microsoft.FSharp.Core.Operators.string

--------------------
type string = System.String

Full name: Microsoft.FSharp.Core.string

val builtin : f:(Exp list -> Exp) -> Exp

Full name: Script.builtin

val f : (Exp list -> Exp)

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More information

Link:	http://fssnip.net/19
Posted:	14 years ago
Author:	Alex Muscar
Tags:	haskell functions