F# Zen – The Literal Attribute
When pattern matching it is easy to forget that you are capturing a new value instead of matching against an existing one. Take this function for example:
let E = 2.718281828459
let PI = 3.141592653589
// Ooops - this captures a value
let isConstant x =
match x with
| PI
| E -> true
| _ -> false
The right way to write this code is to use the [<Literal>] attribute. This tells the F# compiler to treat this value as a constant literal which, among other things, enables it to be used with pattern matching.
[<Literal>]
let E = 2.718281828459
[<Literal>]
let PI = 3.141592653589
// This matches against the literal value
let isConstant x =
match x with
| PI
| E -> true
| _ -> false
