Variables

MSE script has the notion of ''variables''.
A variable is a name holding a value, assigned using the @:=@ operator:
> variable := 1 + 1
From now on (until another value is assigned) @variable@ evaluates to @2@ in the rest of the script.

--Scope--

Assignments to variables are ''local'' to the current function.
Consider:
> function := {
>     x := "something else"
>     # here x is something else
> }
> x := 1
> function()
> # here x is still 1

Unlike most programming languages MSE script uses [[http://en.wikipedia.org/wiki/Dynamic_scoping#Dynamic_scoping|dynamic scoping]].
This means that assignments done in the calling function are visible in the called function:
> fun := { "xyz is {xyz}" }
> one := {
>     xyz := 1
>     fun()
> }
> two := {
>     xyz := "two"
>     fun()
> }
> one()  ==  "xyz is 1"
> two()  ==  "xyz is two"

This can be very useful for variables like @card@, which are used by many functions.

--Overriding functions--
Like custom functions, the [[fun:index|built in functions]] are also stored in variables.
It is possible to overwrite them:
> to_upper := { input }
> to_upper("xyz") == "xyz" # Not what it used to do

A neat trick is adding 'extra' behaviour to functions.
This can be done by first making a copy, and calling that:
> real_to_upper := to_upper
> to_upper := { "upper case: " + real_to_upper() }
> to_upper("xyz") == "upper case: XYZ"
Note that @real_to_upper@ is called without extra parameters, the @input@ variable is still set from the outer call to the new @to_upper@ itself.

<div style="text-align:right;">previous: <a href="operators">&larr; Operators</a> | next: <a href="functions">Functions &rarr;</a></div>