The Expressions DSL

In the DSL we develop in this chapter, which we call Expressions DSL, we want to accept input programs consisting of the following statements: variable declarations with an initialization expression and evaluations of expressions. Variable declarations have the shape var name = exp and evaluation statements have the shape eval exp. Expressions can refer to variables and can perform arithmetic operations, compare expressions, use logical connectors (and and or), and concatenate strings. We will use + both for representing arithmetic addition and for string concatenation. When used with strings, the + will also have to automatically convert integers and Booleans occurring in such expressions into strings.

Here is an example of a program that we want to write with this DSL:

var i = 0
var j = (i > 0 && 1 < (i+1))
var k = 1
eval j || true
eval "a" + (2 * (3 + 5)) // string concatenation
eval (12 / (3 * 2))

For example, "a" + (2 * (3 + 5)) should...