Lesson 20 · Parsing a whole program - Build a Programming Language

Parsing a whole program

Your parser now covers every statement and expression the language has so far. Today you point it at a multi-statement program and wire it into the REPL, which prints the parsed tree back as normalized source.

The goal

Parse a multi-statement program and have the REPL print the whole tree back as source.

Start here - the target

TO DO

Scenario: Parsing and printing a multi-statement program

Giventhe source: let x = 5; let y = x * 2; if (y > 5) { y }

Whenthe parser parses the program

Thenthe program has exactly three statements

Andthe REPL prints the program back as let x = 5;let y = (x * 2);if (y > 5) y

Background

This closes the parsing chapter by running the whole parser at once. A real program is a sequence of statements, and Program.String() just concatenates each statement’s printed form. Parsing let x = 5; let y = x * 2; if (y > 5) { y } should yield exactly three top-level statements, with the middle one’s value correctly grouped as (x * 2).

Wiring this into the REPL upgrades it from a token dumper to a tree printer: type source, and it echoes back the normalized form the parser understood - parentheses made explicit, precedence resolved. That round-trip is the clearest proof the parser is correct, and it is the input the next chapter’s evaluator will finally run.

Make it work

// Program.String() concatenates each statement's String()
func (p *Program) String() string {
  var out strings.Builder
  for _, s := range p.Statements { out.WriteString(s.String()) }
  return out.String()
}
// in the REPL, replace token-printing with: parse, then print program.String()

CheckpointDONE

The REPL parses whatever you type and prints the syntax tree back as source - the parser is complete. Commit and stop for today.