Week 4 Studio: Strings, concatenation, and a working evaluator

String literals and concatenation by juxtaposition, wiring weeks 1–4 into one evaluator that runs real toy SNOBOL-subset programs

15 March 2027

Before the session#

Bring your week 3 evaluator: lexer, parser, symbol table, and a tree-walking evaluator for arithmetic and variables. This session adds the last piece Movement I needs.

In the session#

• Lex string literals. Add quoted-string tokens to the lexer alongside the existing integer and identifier tokens.
• Add a concatenation rule to the parser. Two operands with a blank between them and no operator token is concatenation, at lower precedence than +/-/*// — this slots in as the lowest level of whatever precedence table your Pratt parser from week 2 already has.
• Extend the evaluator’s value type. A value variant with (at least) Int and String constructors, with numeral-string coercion where arithmetic is expected, covers this week’s ground without a separate string-handling path.
• Run real programs. Assemble three or four short toy programs exercising everything from weeks 1–4 together — a literal, an arithmetic expression, an assignment referencing an earlier variable, a concatenation of a string and a coerced number — and confirm each produces the value you expect by hand.

Sample transcript#

Genuine captured output from this course’s own checkpoint-1 reference solution, run against three example programs — not a mockup:

$ dune exec bin/main.exe -- examples/hello.sno
HELLO, SNOBOL4!

$ dune exec bin/main.exe -- examples/arith.sno
14
20
3

$ dune exec bin/main.exe -- examples/blanks.sno
2
5-3

blanks.sno is the one worth tracing by hand before the session: it is built specifically to show that a blank can mean concatenation in one position and nothing at all in another, depending on what’s adjacent to it — the same blank-sensitivity week 2’s lecture introduced.

Afterwards#

Checkpoint 1 is due at the end of this week and asks you to complete a partially-implemented starter covering exactly this material. Keep your working evaluator from this session — it’s the reference you’ll check the checkpoint starter’s expected behaviour against, not a separate piece of work.