Fsharp: Variable

Variable (constants)

all variables are constants (value cannot change), except if declared with keyword mutable.

let xs = "abc"
printfn "%s" xs
// abc

// multiple variables
let xx, yy = (3,4)
printfn "%d %d" xx yy
// 3 4

Mutable variable

let mutable xx = "aa"
printfn "%s" xx
// aa

// change the variable value
xx <- "bb"

printfn "%s" xx
// bb

Named Literal (Declare Compile Time Literal)

• Variables can be declared as compile time constants.
• Variable without mutable declared are always constants, but at run time.

precede the let line by

[<Literal>]

// compile time constant
[<Literal>]
let Xy = 3 + 4
printfn "%d" Xy
// 7

// normal. (run-time constant)
let xx = 3 + 4
printfn "%d" xx
// 7

🟢 TIP: by convention, compile-time constants should start with Capital letter, because in pattern matching, lower case names are always treated as matched patterns.