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         


        

Answer 1

There are two main issues raised by the code you are trying to use.

Firstly pattern matching:

match arg with
| _ -> "value"

matches arg with anything and then returns "value"

match arg with
| a -> "value"

matches arg with anything, calls it "a", and then returns "value". You can think of the match having it's own little namespace, the a only exists in the match, and the match only 'sees' the things that have been named in it.

Secondly, if you want to match to a set of predefined values, then you probably want to use discriminated unions. You can define one like this:

type Keys=
| Key1
| Key2

Then match like this:

match arg with
| Key1 -> "value1"
| Key2 -> "value2"

In this case it matches against the type Keys, not a value called Key1 or Key2.