Add functionality to parse expressions

[keon-dev]

• Before expression parsing the AST must be prepared to accept Expression Statements
• We will be implementing top down operator precedence (or Pratt parsing)

Expressions in monkey

• Monkey has expressions involving prefix operators:
  • -5
  • !true
  • !false
• Monkey has expressions involving infix/binary operators:
  • 5 + 5
  • 5 - 5
  • 5 / 5
  • 5 * 5
• Besides basic arithmetic operators, there are comparison operators:
  • foo == bar
  • foo != bar
  • foo < bar
  • foo > bar
• We can use parentheses to group expressions and influence the order of evaluation:
  • 5 * (5 + 5)
  • ((5 + 5) * 5) * 5
• Monkey has call expressions:
  • add(2, 3)
  • add(add(2, 3), add(5, 10))
  • max(5, add(5, (5 * 5)))
• Identifiers are also expressions in Monkey:
  • foo * bar / foobar
  • add(foo, bar)
• Functions literals are expressions as well, we can use a let statement to bind a function to a name. the function literal is just the expression in the statement:
  • let add = fn(x, y) { return x + y; };
• Function literals can be used instead of identifiers too:
  • fn(x, y) { return x + y }(5, 5)
  • (fn (x) { return x }(5) + 10) * 10
• In monkey there are also "if-expressions":
  • let result = if (10 > 5) { true } else { false }; // => true

[keon-dev]

Each token type will have up to two parsing functions associated with is, depending on whether the token is found in a prefix or an infix position.

type (
    prefixParseFn func() ast.Expression
    infixParseFn  func(ast.Expression) ast.Expression .
)

Both functions return an ast.Expression, but infixParseFn takes another ast.Expression which is the left side of the operator being parsed.