Implement Java Compiler Plugin for witness resolution verification

Author: Garciat

.idea/compiler.xml

@@ -0,0 +1,35 @@
+package com.garciat.typeclasses.processor;
+
+import java.util.List;
+
+import static com.garciat.typeclasses.api.TypeClass.Witness.Overlap.OVERLAPPABLE;
+import static com.garciat.typeclasses.api.TypeClass.Witness.Overlap.OVERLAPPING;
+
+public final class OverlappingInstances {
+  private OverlappingInstances() {}
+
+  /**
+   * @implSpec <a href=
+   *     "https://ghc.gitlab.haskell.org/ghc/doc/users_guide/exts/instances.html#overlapping-instances">6.8.8.5.
+   *     Overlapping instances</a>
+   */
+  public static List<WitnessConstructor> reduce(List<WitnessConstructor> candidates) {
+    return candidates.stream()
+        .filter(
+            iX ->
+                candidates.stream().filter(iY -> iX != iY).noneMatch(iY -> isOverlappedBy(iX, iY)))
+        .toList();
+  }
+
+  private static boolean isOverlappedBy(WitnessConstructor iX, WitnessConstructor iY) {
+    return (iX.overlap() == OVERLAPPABLE || iY.overlap() == OVERLAPPING)
+        && isSubstitutionInstance(iX, iY)
+        && !isSubstitutionInstance(iY, iX);
+  }
+
+  private static boolean isSubstitutionInstance(
+      WitnessConstructor base, WitnessConstructor reference) {
+    return Unification.unify(base.returnType(), reference.returnType())
+        .fold(() -> false, map -> !map.isEmpty());
+  }
+}

.idea/encodings.xml

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+package com.garciat.typeclasses.processor;
+
+import javax.lang.model.type.DeclaredType;
+import javax.lang.model.type.PrimitiveType;
+import javax.lang.model.type.TypeMirror;
+import javax.lang.model.type.TypeVariable;
+
+public sealed interface ParsedType {
+  record Var(TypeVariable java) implements ParsedType {}
+
+  record App(ParsedType fun, ParsedType arg) implements ParsedType {}
+
+  record ArrayOf(ParsedType elementType) implements ParsedType {}
+
+  record Const(DeclaredType java) implements ParsedType {}
+
+  record Primitive(PrimitiveType java) implements ParsedType {}
+
+  default String format() {
+    return switch (this) {
+      case Var v -> v.java.toString();
+      case Const c ->
+          c.java().asElement().getSimpleName()
+              + c.java().getTypeArguments().stream()
+                  .map(TypeMirror::toString)
+                  .reduce((a, b) -> a + ", " + b)
+                  .map(s -> "[" + s + "]")
+                  .orElse("");
+      case App a -> a.fun.format() + "(" + a.arg.format() + ")";
+      case ArrayOf a -> a.elementType.format() + "[]";
+      case Primitive p -> p.java().toString();
+    };
+  }
+}