Variables and Scope

Understand variables and the scoping system in Pile

Variables

In Pile, a variable is a value bound to a name at runtime. When used, its stored value is pushed on the stack.

A local variable can:

Defining and using variables

There are two ways to define variables in Pile, you can use the more compact let statement, or you may use the as-let statement to define multiple variables.

Using let statement

# Use let <name> syntax to define variables
# Remember to be sure that you have at least a value on the stack
# or you will get a Stack Underflow error
69 let x

# Now, to use it, just refer to its name and its value will be pushed onto the stack:
x println # Result: 69

Using as-let statement

as-let statement is a way I found to make defining multiple variables easier through the logic of a program.

Use the as keyword followed by the names of the variables you want to define, separated by space.

69 420 1336 as variable1 variable2 variable3 let
# Note that the binding order is related to how they appear reading, not the order on the top of the stack.
# That means variable1 = 69, variable2 = 420, and variable3 = 1336

Scoping

In Pile, scoping works pretty much like any other programming language. All variables defined inside a scope are invalidated after the end of the same.

The following structures in Pile generate scoped blocks:

Other blocks like conditionals and loops do not create scoped blocks. This is purely a design choice.

Code examples

Here's another example, this time showing how as-let can be used in procedures to work as named arguments:

proc square as number let
  number dup *
end

8 square println
# Output: 64

In this example, the variable number is not valid after the call of square which is (in most cases) the desired behavior for named arguments.