Lesson 01 · A REPL that echoes - Build a Programming Language

A REPL that echoes

Every interpreter needs a way to feed it source and see a result. Today you build the read-eval-print loop that will be your window into the language for the entire project - for now it just echoes, but it is the runnable program everything else plugs into.

The goal

Build a prompt loop that reads a line of input and prints it straight back.

Start here - the target

TO DO

Scenario: The REPL echoes a line of input

Giventhe program is running and shows a prompt

Whenthe user types the line hello world and presses enter

Thenthe program prints hello world

Andit shows the prompt again, ready for the next line

Background

An interpreter is a pipeline - source text goes in one end, tokens become a tree, the tree becomes a value that comes out the other end. You will build that pipeline one stage at a time, but you need a way to drive it from day one. That driver is a REPL: read a line, evaluate it, print the result, loop.

Right now there is nothing to evaluate, so the “eval” step is just an echo. That is fine - the point today is a real, runnable program with a prompt you can type into. Every lesson from here thickens the middle of this loop, and you will run this same program to watch the language come alive.

Make it work

// read lines until end-of-input, echo each one
for {
  fmt.Print(">> ")
  line, ok := readLine()
  if !ok { break }   // Ctrl-D / EOF ends the loop
  fmt.Println(line)
}

CheckpointDONE

You have a running program with a prompt that reads and prints lines. Commit and stop for today.