src/rules/correctness/chars.bend source

src/rules/correctness/chars.bend on the hub · documented module

# 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.import Baseimport ../../src.bend as Srcimport ../../finding.bend as Fimport ../../syntax/lex.bend as Leximport ../../syntax/tree.bend as Treeimport ../../lazy/lazy.bend as Lazyimport ../tokens.bend as T# one when a case pattern opens with a character literal, else zerodef literal(kids: Tree.Node) -> U32:  match kids:    case Tree.NCons{Tree.Leaf{Lex.Tok{Lex.TChar{}, t, l, c}}, rest}:      1    case other:      0# the character-literal arms among a match's statementsdef tally(body: Tree.Node, +nn: U32) -> U32:  match body:    case Tree.NCons{Tree.Stmt{Tree.SCase{}, kids, b}, rest}:      +k = literal(T.pattern(kids))      tally(rest, (nn + k : U32))    case Tree.NCons{h, rest}:      tally(rest, nn)    case other:      nn# a finding when the match holds more character literals than is cheapdef report(+nn: U32, +path: String, +ll: U32, +cc: U32) -> List<&2, F.Finding>:  Bool.pick(List<&2, F.Finding>, U32.is_gt(nn, 8),    [F.Finding{path, ll, cc, 5, "chars", "This match has " ++ U32.show(nn)      ++ " character-literal cases, over the limit of 8, which blows up compile time and memory; compare Char.to_u32(c) instead."}],    [])# a statement: a match is tallieddef on_stmt(+kids: Tree.Node, +body: Tree.Node, +path: String) -> List<&2, F.Finding>:  +hit = T.keyword(kids, "match")  Lazy.stop(List<&2, F.Finding>, Bool.not(hit), [],    _u => report(tally(body, 0), path, Tree.line(kids), Tree.col(kids)))# every statement, at any depthdef walk(nn: Tree.Node, +path: String) -> List<&2, F.Finding>:  match nn:    case Tree.NCons{Tree.Stmt{kind, kids, +body}, rest}:      List.concat(&2, F.Finding, [on_stmt(kids, body, path), walk(body, path), walk(rest, path)])    case Tree.NCons{h, rest}:      walk(rest, path)    case other:      Nil{}# the ruledef check(ss: Src.Src) -> List<&2, F.Finding>:  Src.Src{path, text, toks, tree, bound, items} = ss  walk(tree, path)