A variable always has to defined prior to its first use. A new ASM variable can be defined by a let rule, a choose rule, a forall rule, or as a formal parameter of an ASM rule.

ASM variables are untyped at compile-time, i.e. we cannot assign types at compile-time. Thus the type of a variable is induced at run-time, and this run-time type may change during execution, i.e. the same variable may store once an integer value and then a string, for instance, but, of course, it is not a good practice for writing transformations.

Variables can contain constants of the following types: string, integer, float, boolean, and model elements.

The scope of an ASM variable can be defined according to the place where the variable was defined first, thus it can be rule-scopeor block-scope. ASM rules that define variables with block-scope are the let rule, the forall ruleand the choose rule.

Each variable is visible and accessible anywhere inside its scope including sub-blocks at arbitrary depth, but it becomes undefined outside its scope. As a result, ASM variables that are undefined within a certain scope cause compilation errors.

A variable can be defined only by the let rule, choose ruleand forall ruleor in a formal parameter list ( formal params, directed formal params).