Week 4: Strings without a plus sign

Strings as first-class values, and concatenation by juxtaposition — no operator token, adjacency is the operator

15 March 2027

Outline

Concatenation has no operator

This is a fact worth stating plainly because it is easy to disbelieve until you’ve read it in the primary source: SNOBOL4 has no concatenation operator. Where most languages write a + b or a . b or a ++ b to join two strings, SNOBOL4 writes them adjacent, separated only by a blank. TENS UNITS is not two operands waiting for an operator — the blank is the operator, in the sense that two blank-separated string-valued operands with no operator token between them are silently concatenated. Combined with week 2’s blank-sensitivity rule, this gives concatenation the lowest precedence of any operation in the language: arithmetic and comparison bind first, and whatever’s left is concatenated.

This has a direct, practical consequence for your parser: (TENS UNITS) 30 concatenates TENS and UNITS first, then treats the four-character result as one operand — the parentheses are load-bearing, not decorative, because without them the grammar for what looks like a pattern-matching subject would parse differently than intended. A compiler for this language needs to treat “no token here, just a blank, and it’s not one of the unary-adjacency cases from week 2” as itself a meaningful parse decision, not an absence of one.

Strings as ordinary values

Nothing about strings requires special-casing once concatenation is handled: they participate in the same variable-assignment and expression machinery as integers, and “numeral strings” — strings that look like a number — coerce into arithmetic contexts automatically, the same coercion rule from week 3’s null-string discussion. The practical implication for this week’s evaluator is that a value variant with a String and an Int (and, eventually, Real) constructor, plus one concatenation case in the evaluator, covers everything Movement I asks of strings — there is no separate “string mode” to design around.

Movement I, assembled

By the end of this week your interpreter should evaluate all of Movement I’s material in one pass: integer and string literals, arithmetic with correct precedence, variable assignment and lookup through a flat symbol table, and string concatenation by juxtaposition. That is deliberately the whole of “a calculator that talks back” — nothing here is provisional or due for a rewrite in Movement II, which instead builds statements, the goto field, and control flow on top of what you have now. Checkpoint 1, due at the end of this week, asks you to complete a starter implementation covering exactly this ground.

Further reading

Teaching team