F# pattern match directly against let binding

Question

Is it possible in F# to pattern match directly against a let binding?

For example, this compiles without any warnings:

    let value = 
        match arg with
        | 1 -> "value1"
        | 2 -> "value2"
        | _ -> failwith "key not found"

Whereas the following gives the warning "This rule will never be matched" against the lines matching key2 and _:

    let key1 = 1
    let key2 = 2
    let value = 
        match arg with
        | key1 -> "value1"
        | key2 -> "value2"
        | _ -> failwith "key not found"

Is this because although they're immutable, the let bindings are unlike C# const variables?

Answer 1

just use capital letters and [<Literal>] them and it works as expected.

let [<Literal>] X = 0
let [<Literal>] Y = 1
let bla arg =
    match arg with
    | X -> "zero"
    | Y -> "one"
    | somethingelse -> somethingelse.ToString()

the lower case name by convention typically means a wildcard that is bound to the name.