Week 3 Studio: Assignment and a symbol table

Implementing assignment statements over a single flat symbol table — variables as SNOBOL4 actually has them, not as most languages have them

8 March 2027

Before the session#

Bring week 2’s parser and evaluator. This week extends the expression grammar with variable references and adds the first statement form.

In the session#

• Add variable tokens. A bare identifier is a variable reference. Extend the parser so an identifier can appear anywhere a literal currently can.
• Build the symbol table. One mutable table, name to value, built once and shared for the whole run — resist the urge to reach for anything scope-chain-shaped. That impulse is exactly the mainstream-language intuition this week’s lecture asks you to set aside; a flat table is not a shortcut here, it’s the correct model.
• Implement assignment. subject = object updates the subject’s cell in the symbol table to the object’s evaluated value. Real SNOBOL4 has no = token at all — plain assignment is just subject blank object, with = reserved for a replacement statement’s object field — but this course’s subset keeps = throughout, matching checkpoint 1’s already-established surface syntax rather than introducing a second assignment notation this late. A reference to a variable that has never been assigned should evaluate to the null string rather than raising an error — decide now how your value type represents that, since it’s the same question week 4’s strings answer more fully.
• Check it end to end. A short sequence of assignments and expressions referencing earlier ones (X = 5, Y = X * 2 + 1) should evaluate correctly in order.

Sample transcript#

Genuine captured output from this course’s own week-3 reference evaluator, run against a short sequence of assignments:

$ dune exec bin/main.exe -- examples/vars.sno
11

9

The blank middle line is worth tracing by hand, not a rendering glitch: Z = UNSET assigns Z the value of UNSET, a variable that has itself never been assigned, so it evaluates to the null string per this week’s spec — that null string is what gets printed, not an error.

This week’s real starter — the symbol table wired up, with assignment and lookup themselves stubbed out — is downloadable at week-03-starter.zip.

Afterwards#

The symbol table you build this week is the one the rest of the course reuses without redesigning it — this is the one place “everything is global” pays off as a simplification rather than a constraint.