Checkpoint 1

Complete a partially-implemented OCaml starter — literals, arithmetic, variables, and string concatenation — matching Movement I's four sessions

Due: 19 March 2027 · Weight: 20%

The brief#

Complete a starter OCaml implementation so it evaluates literals, arithmetic, variable assignment and lookup, and string concatenation — the four things Movement I’s sessions built up over weeks 1–4.

You are not starting from a blank page, and you are not being handed a finished solution. You get real starter code: a working hand-rolled lexer, and a parser and evaluator with the Movement I gaps marked as failwith "TODO". Your job is to fill in exactly those gaps, using the same representation choices the starter already commits to — a value variant type, a flat symbol table, and a Pratt-style expression parser — rather than redesigning the pipeline. Download the starter at checkpoint-1-starter.zip; it includes a sample transcript showing the exact input/output behaviour your completed evaluator should match once every TODO is filled in.

What makes a strong submission here is narrow, on purpose: this is a correctness checkpoint, not an open brief. A response that reproduces the sample transcript’s behaviour on the given inputs, and behaves sensibly on inputs of the same shape the transcript didn’t happen to cover (a longer arithmetic expression, a variable referencing another variable, a concatenation involving a numeral string), is a complete response. Anything beyond Movement I’s four topics — pattern matching, the goto field, function definitions — is out of scope for this checkpoint; it belongs to later movements.

What you submit#

Your completed .ml/.mll source files, in the same project layout the starter arrived in, building cleanly with the starter’s own dune setup. Include a short note (a few sentences is enough) naming any place you departed from the starter’s existing structure and why — this is the “quality of execution” half of the criteria below, and it’s also where you declare any assistance you used, per the course’s academic integrity policy.

The marking: block above renders as a weighted criterion table: the bulk of the mark is whether your evaluator actually reproduces the sample transcript’s behaviour; the remainder is whether your completed code reads as a reasonable continuation of the starter rather than a bolt-on.