In this section, the definitions are stored in files.
In Eq all variables are defined at the top level and followed by a ;. You cannot redefine a variable :
$ eq eval 'a := 3; a :=3'; true
a is already defined
a
a := 3
Two define a variable, you can use two operators, the first one and simpler the attribution operator (same used in previous example)
$ cat docexample/var_bind.txt
x := 3;
x * x * x
$ eq eval -f docexample/var_bind.txt
27
The := operator let you set the value of a variable. The right and side of the operator (3 here), is evaluated at the definition site.
The ; separate all your definitions. Only the value of the last expression is displayed. During the evaluation of an expression if a variable is found, we check if it’s bound, if it is, we replace the variable by the bound value and continue evaluation.
When a variable is found in a formula, a previous definition is searched for it. If one exists, the variable in the formula is substituted by it’s definition. Once the substitution is done, the definition of the variable is evaluated.
If no definition exist for the variable, the variable stay as-is, with no modification.
The difference between the := (attribution) and :> (lazy attribution) operator reside in the evaluation of the right hand side. The attribution operator evaluate it, whereas the lazy one doesn’t. It can be really important for the definition of some recursive functions.