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src/Draco.Compiler/Internal/Lowering/LocalRewriter_MatchExpression.cs
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,97 @@ | ||
| using System; | ||
| using System.Collections.Generic; | ||
| using System.Collections.Immutable; | ||
| using System.Linq; | ||
| using System.Text; | ||
| using System.Threading.Tasks; | ||
| using Draco.Compiler.Internal.BoundTree; | ||
| using Draco.Compiler.Internal.Symbols; | ||
| using static Draco.Compiler.Internal.BoundTree.BoundTreeFactory; | ||
|
|
||
| namespace Draco.Compiler.Internal.Lowering; | ||
|
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| internal sealed partial class LocalRewriter | ||
| { | ||
| public override BoundNode VisitMatchExpression(BoundMatchExpression node) | ||
| { | ||
| // match (matchedExpr) { | ||
| // pattern1 if (guard1) -> value1; | ||
| // pattern2 if (guard2) -> value2; | ||
| // ... | ||
| // } | ||
| // | ||
| // => | ||
| // | ||
| // { | ||
| // val tmp = matchedExpr; | ||
| // if (matches-pattern1(tmp) and guard1) value1 | ||
| // else if (matches-pattern2(tmp) and guard2) value2 | ||
| // ... | ||
| // } | ||
|
|
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| // Evaluate the matched value as a local to not duplicate side-effects | ||
| var matchedValue = (BoundExpression)node.MatchedValue.Accept(this); | ||
| var tmp = this.StoreTemporary(matchedValue); | ||
|
|
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| var conditionValuePairs = new List<(BoundExpression Condition, BoundExpression Value)>(); | ||
| foreach (var arm in node.MatchArms) | ||
| { | ||
| var patternMatcher = this.ConstructPatternToMatch(arm.Pattern, tmp.Reference); | ||
| var guard = arm.Guard ?? this.LiteralExpression(true); | ||
|
|
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| var condition = AndExpression(patternMatcher, guard); | ||
|
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| conditionValuePairs.Add((condition, arm.Value)); | ||
| } | ||
|
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| // NOTE: We do an r-fold to nest if-expressions to the right | ||
| conditionValuePairs.Reverse(); | ||
| // TODO: The match-expr might be empty! | ||
| var lastPair = conditionValuePairs[0]; | ||
| var ifChain = (BoundExpression)conditionValuePairs | ||
| .Skip(1) | ||
| .Aggregate( | ||
| IfExpression( | ||
| lastPair.Condition, | ||
| lastPair.Value, | ||
| BoundTreeFactory.LiteralExpression(null, node.TypeRequired), | ||
| node.TypeRequired), | ||
| (acc, arm) => IfExpression(arm.Condition, arm.Value, acc, node.TypeRequired)) | ||
| .Accept(this); | ||
|
|
||
| return BlockExpression( | ||
| locals: tmp.Symbol is null | ||
| ? ImmutableArray<LocalSymbol>.Empty | ||
| : ImmutableArray.Create(tmp.Symbol), | ||
| statements: ImmutableArray.Create(tmp.Assignment), | ||
| ifChain); | ||
| } | ||
|
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| private BoundExpression ConstructPatternToMatch(BoundPattern pattern, BoundExpression matchedValue) => pattern switch | ||
| { | ||
| BoundDiscardPattern discard => this.ConstructPatternToMatch(discard, matchedValue), | ||
| BoundLiteralPattern literal => this.ConstructPatternToMatch(literal, matchedValue), | ||
| _ => throw new ArgumentOutOfRangeException(nameof(pattern)), | ||
| }; | ||
|
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| private BoundExpression ConstructPatternToMatch(BoundDiscardPattern pattern, BoundExpression matchedValue) => | ||
| // _ matches expr | ||
| // | ||
| // => | ||
| // | ||
| // true | ||
| this.LiteralExpression(true); | ||
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| private BoundExpression ConstructPatternToMatch(BoundLiteralPattern pattern, BoundExpression matchedValue) => | ||
| // N matches expr | ||
| // | ||
| // => | ||
| // | ||
| // object.Equals(N, expr) | ||
| CallExpression( | ||
| receiver: null, | ||
| method: this.WellKnownTypes.SystemObject_Equals, | ||
| arguments: ImmutableArray.Create( | ||
| this.LiteralExpression(pattern.Value), | ||
| matchedValue)); | ||
| } |