Back to Questions
744
views1
comment
How to declare a named constant for use in patterns?
f#

I would like to use symbolic constants when doing pattern matching. E.g. instead of

1
2

match x with
 | Foo("very_long_string_constant_value_here") ..

I would like to write something like

1
2
3
4

let val = "very_long_string_constant_value_here";
...
match x with
 | Foo(val)

However, if I do it like this, there will be no matching. Instead, the matching engine will capture the parameter of Foo and bind it to a new variable 'val', which will hide the global 'val'.

F# manual says that

".NET named constant literals may be used as constants within patterns"

How do I declare .NET named constant literal in F#?

.

The way to write that is using the [<Literal>] attribute.

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15

#light

[<Literal>]
let One = 1

<Literal>]
let Three = 1

let test x = 
    match x with
    | One -> printfn "one"
    | Three -> printfn "three"
    | x -> printfn "%d" x

[1 .. 5] |> List.iter test.

However, this feature was added recently and is not in the most recent release.

It will however be in the next release due out sometime soonish

By on 6/17/2008 10:18 AM ()

    Topic tags

    IntelliFactory Offices Copyright (c) 2011-2012 IntelliFactory. All rights reserved.
    Home | Products | Consulting | Trainings | Blogs | Jobs | Contact Us | Terms of Use | Privacy Policy | Cookie Policy     		Built with WebSharper