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5: Local Variables

As with any other programming language, you can declare variables inside a function using the local keyword:1

(local $my_value i32)

Here, we have declared that within a function (not shown here) we have a local variable of type i32 called $my_value.

IMPORTANT

  1. Variable names must start with a dollar sign
  2. Local variables are automatically initialised to zero

Now that we have a local variable, we can store a value in it:

i32.const 5              ;; Push a value onto the stack
local.set $my_value      ;; Pop the value off the stack and store it in the named variable

Now that variable $my_value contains 5, we can use local.get to push a copy of that value back onto the stack:

local.get $my_value      ;; Stack = [5]

Naming Local Variables

Strangely enough, you do not need to supply a name when declaring either a local variable or a function.

This might sound pretty weird, but the point is that human-readable names are only of benefit to humans. In a WAT program, it’s perfectly possible to declare variables like this:

(local i32 i32 f64)  ;; Declare three unnamed variables, two i32s and an f64

Uh, OK… So how do you reference these local values?

Using Index Numbers to Reference Variables and Functions

Internally, WebAssembly always refers to variables and functions using index numbers.

Within the scope of a module, functions are identified using zero-based index numbers on the basis of the order in which the function declarations are encountered.

Within the scope of a function, variables are identified using zero-based index numbers on the basis of their declaration order.

Assuming these are the first local variables declared in a function, then the two i32s will be variables 0 and 1, and the f64 will be variable 2.

The point here is that even if you do not assign a human-readable name to a local variable, that variable can always be accessed using its index number.

So if you want to store 5 in the second of your local variables (variable 1), it is quite acceptable to write:

(local.set 1 (i32.const 5))    ;; Store 5 in local variable 1

The problem is, you now need to remember what variable 1 holds. And this is where humans rapidly begin to struggle, because once we get beyond a small number of abstract tokens, we simply can’t remember what they mean.

Us humans need meaningful variable names — so let’s keep using them!

  1. You can also declare variables that are global to the scope of the entire module, but we won’t worry about these for the time being ↩