src/rules/correctness/chars.bend fails

raw source on the hub · import 0xde9bb08f7de298b03207fb5797ede9a5/src/rules/correctness/chars.bend as Chars

rule chars: a match with more than eight character-literal arms (case '.':). Char is Chr{code: U32}, so such an arm is a U32 literal inside a constructor pattern, and bend's C backend pays for each one out of all proportion to its size -- about 90 MB, and the cost compounds through every def downstream of the one holding them (eighteen arms cost 1.57 GB and 8.3 s here, 0.10 GB and 0.7 s once rewritten). It is the literals, not the arms: a match over eighteen constructors costs nothing measurable.

Compare Char.to_u32(c) instead. Bind the fallback above the comparisons so it is a value, not a call, or bolt's own eager rule fires on it; and give the def the reusable quantity (+c: Char), since the code point and the fallback both consume it. Where the arms carry linear values, leave the match alone: a cascade would break linearity and do every branch's work.

Only a case's first match column is read, and only an arm whose pattern opens with a character literal counts: case 'x': does, case Con{'x', t}: and a literal in a later column of match a b: do not.

7 imports

import Base
import ../../src.bend as Src
import ../../finding.bend as F
import ../../syntax/lex.bend as Lex
import ../../syntax/tree.bend as Tree
import ../../lazy/lazy.bend as Lazy
import ../tokens.bend as T

The reader could not load this file (at 0xde9bb08f7de298b03207fb5797ede9a5/src/config.bend:32). What bend.ts says:

Error:
- expected : a fresh name (duplicate declaration: Set)
- observed : 'Set'
Location:
31 | # a name (a group's or a rule's) at a level
32>| type Set is Data:
   |      ^^^
33 |   Set{name: String, level: Level}