Appearance
| 1 | namespace Syntax.Process is | |
| 2 | use Logging | |
| 3 | ||
| 4 | use Semantic.Types.Type | |
| 5 | ||
| 6 | // Exhaustiveness / redundancy check over a `case` statement or | |
| 7 | // expression. The check runs once after every arm's pattern and | |
| 8 | // statements have been compiled, so the scrutinee's type and each | |
| 9 | // arm's classification are settled. | |
| 10 | // | |
| 11 | // The arms are read as rows of patterns and put to | |
| 12 | // CASE_USEFULNESS, which answers both questions the construct | |
| 13 | // needs: the arms are exhaustive when a row of wildcards matches | |
| 14 | // nothing they leave open, and an arm is redundant when its own | |
| 15 | // row matches nothing the arms before it left open. Whichever | |
| 16 | // question is being asked, the answer comes back as the value | |
| 17 | // shapes that escape, so a diagnostic can name them. | |
| 18 | // | |
| 19 | // A scrutinee whose values cannot be enumerated at all — `int`, | |
| 20 | // `string`, an open class hierarchy — has no domain to cover, and | |
| 21 | // takes the `case-needs-else` branch instead: a warning on the | |
| 22 | // statement form and on an expression form with a defaultable | |
| 23 | // expected type, an error otherwise. | |
| 24 | class CASE_EXHAUSTIVENESS_CHECKER is | |
| 25 | _logger: Logger | |
| 26 | _classifier: CASE_DOMAIN_CLASSIFIER | |
| 27 | _builder: CASE_PATTERN_BUILDER | |
| 28 | _usefulness: CASE_USEFULNESS | |
| 29 | ||
| 30 | // How many missing cases a diagnostic names before it stops. | |
| 31 | // A sum type lists its uncovered alternatives and there are | |
| 32 | // rarely many; a product can leave a great many combinations | |
| 33 | // open, and listing them all would bury the point. | |
| 34 | _MAX_REPORTED_WITNESSES: int static => 8 | |
| 35 | ||
| 36 | init( | |
| 37 | logger: Logger, | |
| 38 | innate_symbol_lookup: Semantic.Lookups.InnateSymbolLookup | |
| 39 | ) is | |
| 40 | super.init() | |
| 41 | ||
| 42 | _logger = logger | |
| 43 | _classifier = CASE_DOMAIN_CLASSIFIER(innate_symbol_lookup) | |
| 44 | _builder = CASE_PATTERN_BUILDER() | |
| 45 | _usefulness = CASE_USEFULNESS() | |
| 46 | si | |
| 47 | ||
| 48 | check(`case: Trees.Statements.CASE) is | |
| 49 | let expression_value = `case.expression?.value | |
| 50 | ||
| 51 | if !expression_value? then | |
| 52 | return | |
| 53 | fi | |
| 54 | ||
| 55 | let scrutinee_type = expression_value.type | |
| 56 | ||
| 57 | if !scrutinee_type? \/ !scrutinee_type.is_settled then | |
| 58 | return | |
| 59 | fi | |
| 60 | ||
| 61 | let domain = _classifier.classify(scrutinee_type) | |
| 62 | ||
| 63 | if !domain? then | |
| 64 | _check_open_domain(`case, scrutinee_type) | |
| 65 | ||
| 66 | return | |
| 67 | fi | |
| 68 | ||
| 69 | let domains = Collections.LIST[PATTERN_DOMAIN?]() | |
| 70 | ||
| 71 | domains.add(domain) | |
| 72 | ||
| 73 | let rows = Collections.LIST[Collections.List[CASE_PATTERN]]() | |
| 74 | let else_arm: Trees.Statements.CASE_MATCH? mut = null | |
| 75 | ||
| 76 | for arm in `case.matches do | |
| 77 | if _is_else_arm(arm) then | |
| 78 | if !else_arm? then | |
| 79 | else_arm = arm | |
| 80 | fi | |
| 81 | ||
| 82 | continue | |
| 83 | fi | |
| 84 | ||
| 85 | let arm_rows = _builder.build_rows(arm, domain) | |
| 86 | ||
| 87 | // An arm whose shape could not be modelled covers | |
| 88 | // nothing as far as this check is concerned, and is | |
| 89 | // exempt from the redundancy report for the same | |
| 90 | // reason a guarded arm is. | |
| 91 | if !arm_rows? then | |
| 92 | continue | |
| 93 | fi | |
| 94 | ||
| 95 | if !_adds_anything(rows, domains, arm_rows) then | |
| 96 | _logger.warn( | |
| 97 | arm.location, | |
| 98 | "redundant-case-arm", | |
| 99 | "this arm is already covered by a preceding arm" | |
| 100 | ) | |
| 101 | fi | |
| 102 | ||
| 103 | for row in arm_rows do | |
| 104 | rows.add(row) | |
| 105 | od | |
| 106 | od | |
| 107 | ||
| 108 | let witnesses = | |
| 109 | _usefulness.find_witnesses( | |
| 110 | rows, | |
| 111 | domains, | |
| 112 | CASE_PATTERN_BUILDER.wildcards(1), | |
| 113 | _MAX_REPORTED_WITNESSES | |
| 114 | ) | |
| 115 | ||
| 116 | let is_exhaustive = witnesses.count == 0 | |
| 117 | ||
| 118 | if is_exhaustive then | |
| 119 | `case.case_state.is_exhaustive = true | |
| 120 | fi | |
| 121 | ||
| 122 | if else_arm? then | |
| 123 | if is_exhaustive then | |
| 124 | _logger.warn( | |
| 125 | else_arm.location, | |
| 126 | "dead-case-else", | |
| 127 | "all cases are already covered before this else" | |
| 128 | ) | |
| 129 | fi | |
| 130 | ||
| 131 | return | |
| 132 | fi | |
| 133 | ||
| 134 | if is_exhaustive then | |
| 135 | return | |
| 136 | fi | |
| 137 | ||
| 138 | let missing = _describe(witnesses) | |
| 139 | ||
| 140 | // A product scrutinee reports through the same branch an | |
| 141 | // unenumerable one does, naming the combinations it | |
| 142 | // leaves open. Covering a product has never been required | |
| 143 | // of an author, so a gap in one stays a fall-through | |
| 144 | // rather than the error a gap in a sum is. | |
| 145 | if _is_product(domain) then | |
| 146 | _emit_needs_else(`case, scrutinee_type, missing) | |
| 147 | ||
| 148 | return | |
| 149 | fi | |
| 150 | ||
| 151 | let message = "case is not exhaustive: missing {missing}" | |
| 152 | let header = `case.location.start_position :: `case.expression.location | |
| 153 | ||
| 154 | if `case.want_value then | |
| 155 | _logger.error( | |
| 156 | header, | |
| 157 | message | |
| 158 | ) | |
| 159 | else | |
| 160 | _logger.warn( | |
| 161 | header, | |
| 162 | "non-exhaustive-case", | |
| 163 | message | |
| 164 | ) | |
| 165 | fi | |
| 166 | si | |
| 167 | ||
| 168 | // A scrutinee with no enumerable domain is covered only by an | |
| 169 | // arm that cannot fail to match. | |
| 170 | _check_open_domain(`case: Trees.Statements.CASE, scrutinee_type: Type) is | |
| 171 | for arm in `case.matches do | |
| 172 | if arm.guard? then | |
| 173 | continue | |
| 174 | fi | |
| 175 | ||
| 176 | if !arm.expressions? /\ !arm.pattern? then | |
| 177 | return | |
| 178 | fi | |
| 179 | ||
| 180 | if let pattern = arm.pattern then | |
| 181 | if !pattern.is_explicit_type /\ !pattern.left.has_intrinsic_refutability then | |
| 182 | return | |
| 183 | fi | |
| 184 | fi | |
| 185 | od | |
| 186 | ||
| 187 | _emit_needs_else(`case, scrutinee_type, null) | |
| 188 | si | |
| 189 | ||
| 190 | _emit_needs_else(`case: Trees.Statements.CASE, scrutinee_type: Type, missing: string?) is | |
| 191 | let detail = | |
| 192 | if let m = missing then | |
| 193 | "missing {m}" | |
| 194 | else | |
| 195 | "{scrutinee_type} cannot be exhaustively matched" | |
| 196 | fi | |
| 197 | ||
| 198 | let header = `case.location.start_position :: `case.expression.location | |
| 199 | ||
| 200 | if !`case.want_value then | |
| 201 | _logger.warn( | |
| 202 | header, | |
| 203 | "case-needs-else", | |
| 204 | "case without else may fall through: {detail}" | |
| 205 | ) | |
| 206 | ||
| 207 | return | |
| 208 | fi | |
| 209 | ||
| 210 | let expected_type = `case.expected_type | |
| 211 | ||
| 212 | if expected_type? /\ _is_defaultable(expected_type) then | |
| 213 | _logger.warn( | |
| 214 | header, | |
| 215 | "case-needs-else", | |
| 216 | "case without else may return default: {detail}" | |
| 217 | ) | |
| 218 | ||
| 219 | `case.case_state.requires_default_fallthrough = true | |
| 220 | ||
| 221 | return | |
| 222 | fi | |
| 223 | ||
| 224 | _logger.error( | |
| 225 | header, | |
| 226 | "else required: {detail}" | |
| 227 | ) | |
| 228 | si | |
| 229 | ||
| 230 | // True when the arm's rows reach a value none of the rows | |
| 231 | // before it do. | |
| 232 | _adds_anything( | |
| 233 | rows: Collections.List[Collections.List[CASE_PATTERN]], | |
| 234 | domains: Collections.List[PATTERN_DOMAIN?], | |
| 235 | arm_rows: Collections.List[Collections.List[CASE_PATTERN]] | |
| 236 | ) -> bool is | |
| 237 | for row in arm_rows do | |
| 238 | if _usefulness.find_witnesses(rows, domains, row, 1).count > 0 then | |
| 239 | return true | |
| 240 | fi | |
| 241 | od | |
| 242 | ||
| 243 | return false | |
| 244 | si | |
| 245 | ||
| 246 | _describe(witnesses: Collections.List[Collections.List[CASE_PATTERN]]) -> string static => | |
| 247 | witnesses | |
| 248 | |> Ghul.Pipes.map(w => w[0].describe()) | |
| 249 | |> Ghul.Pipes.join(", ") | |
| 250 | ||
| 251 | _is_product(domain: PATTERN_DOMAIN) -> bool static => | |
| 252 | domain.constructors.count == 1 /\ | |
| 253 | domain.constructors[0].kind == ConstructorKind.PRODUCT | |
| 254 | ||
| 255 | _is_defaultable(t: Type) -> bool static => | |
| 256 | t.is_value_type \/ t.is_optional | |
| 257 | ||
| 258 | _is_else_arm(arm: Trees.Statements.CASE_MATCH) -> bool static => | |
| 259 | !arm.expressions? /\ !arm.pattern? | |
| 260 | si | |
| 261 | si |