Support ignore target

15 files changed, 87 insertions, 2 deletions

diff --git a/src/main/java/org/shedlang/compiler/ast/typed/TypedTargetNode.java b/src/main/java/org/shedlang/compiler/ast/typed/TypedTargetNode.java
index 61ead210..1b469175 100644
--- a/src/main/java/org/shedlang/compiler/ast/typed/TypedTargetNode.java
+++ b/src/main/java/org/shedlang/compiler/ast/typed/TypedTargetNode.java
@@ -5,6 +5,7 @@ import org.shedlang.compiler.sources.Source;
 import java.util.List;
 
 public sealed interface TypedTargetNode extends TypedNode {
+    record Ignore(Source source) implements TypedTargetNode {}
     record Reassign(String name, Source source) implements TypedTargetNode {}
     record Tuple(List<TypedTargetNode> elements, Source source) implements TypedTargetNode {}
     record Val(String name, Source source) implements TypedTargetNode {}
diff --git a/src/main/java/org/shedlang/compiler/ast/untyped/UntypedTargetNode.java b/src/main/java/org/shedlang/compiler/ast/untyped/UntypedTargetNode.java
index 3cea8c2d..1d4cbe7e 100644
--- a/src/main/java/org/shedlang/compiler/ast/untyped/UntypedTargetNode.java
+++ b/src/main/java/org/shedlang/compiler/ast/untyped/UntypedTargetNode.java
@@ -5,6 +5,7 @@ import org.shedlang.compiler.sources.Source;
 import java.util.List;
 
 public sealed interface UntypedTargetNode extends UntypedNode {
+    record Ignore(Source source) implements UntypedTargetNode {}
     record Reassign(String name, Source source) implements UntypedTargetNode {}
     record Tuple(List<UntypedTargetNode> elements, Source source) implements UntypedTargetNode {}
     record Val(String name, Source source) implements UntypedTargetNode, UntypedVariableDeclarationNode {}
diff --git a/src/main/java/org/shedlang/compiler/parser/Parser.java b/src/main/java/org/shedlang/compiler/parser/Parser.java
index 44c132e7..eef01d28 100644
--- a/src/main/java/org/shedlang/compiler/parser/Parser.java
+++ b/src/main/java/org/shedlang/compiler/parser/Parser.java
@@ -301,7 +301,7 @@ public class Parser {
 
     static UntypedFunctionStatementNode parseFunctionStatement(TokenIterator<TokenType> tokens) {
         return switch (tokens.peek().tokenType()) {
-            case KEYWORD_ASSIGN, KEYWORD_VAL, KEYWORD_VAR ->
+            case KEYWORD_ASSIGN, KEYWORD_VAL, KEYWORD_VAR, SYMBOL_UNDERSCORE ->
                 parseAssign(tokens);
 
             case KEYWORD_RETURN ->
@@ -633,6 +633,15 @@ public class Parser {
                 yield new UntypedTargetNode.Tuple(elements, source);
             }
 
+            case SYMBOL_UNDERSCORE -> {
+                tokens.skip(TokenType.SYMBOL_UNDERSCORE);
+
+                var end = tokens.position();
+                var source = start.to(end);
+
+                yield new UntypedTargetNode.Ignore(source);
+            }
+
             default ->
                 throw new UnexpectedTokenException("target", tokens.peek());
         };
diff --git a/src/main/java/org/shedlang/compiler/parser/TokenType.java b/src/main/java/org/shedlang/compiler/parser/TokenType.java
index d9cd95aa..53a7877a 100644
--- a/src/main/java/org/shedlang/compiler/parser/TokenType.java
+++ b/src/main/java/org/shedlang/compiler/parser/TokenType.java
@@ -37,4 +37,5 @@ public enum TokenType {
     SYMBOL_PLUS,
     SYMBOL_SEMICOLON,
     SYMBOL_THIN_ARROW,
+    SYMBOL_UNDERSCORE,
 }
diff --git a/src/main/java/org/shedlang/compiler/parser/Tokeniser.java b/src/main/java/org/shedlang/compiler/parser/Tokeniser.java
index 3793417a..bfa75a8e 100644
--- a/src/main/java/org/shedlang/compiler/parser/Tokeniser.java
+++ b/src/main/java/org/shedlang/compiler/parser/Tokeniser.java
@@ -51,7 +51,8 @@ public class Tokeniser {
             RegexTokeniser.rule(TokenType.SYMBOL_PAREN_OPEN, "\\("),
             RegexTokeniser.rule(TokenType.SYMBOL_PAREN_CLOSE, "\\)"),
             RegexTokeniser.rule(TokenType.SYMBOL_PLUS, "\\+"),
-            RegexTokeniser.rule(TokenType.SYMBOL_SEMICOLON, ";")
+            RegexTokeniser.rule(TokenType.SYMBOL_SEMICOLON, ";"),
+            RegexTokeniser.rule(TokenType.SYMBOL_UNDERSCORE, "_")
         )
     );
 
diff --git a/src/main/java/org/shedlang/compiler/stackir/StackCompiler.java b/src/main/java/org/shedlang/compiler/stackir/StackCompiler.java
index d036eebe..07180669 100644
--- a/src/main/java/org/shedlang/compiler/stackir/StackCompiler.java
+++ b/src/main/java/org/shedlang/compiler/stackir/StackCompiler.java
@@ -372,6 +372,10 @@ public class StackCompiler {
         Context context
     ) {
         return switch (target) {
+            case TypedTargetNode.Ignore ignore -> {
+                yield context.addInstruction(new DiscardInstruction(ignore.source()));
+            }
+
             case TypedTargetNode.Reassign reassign -> {
                 context = context.addInstruction(
                     new VarStoreInstruction(
diff --git a/src/main/java/org/shedlang/compiler/typechecker/DeclaredVariables.java b/src/main/java/org/shedlang/compiler/typechecker/DeclaredVariables.java
index 43a087b5..7175f51c 100644
--- a/src/main/java/org/shedlang/compiler/typechecker/DeclaredVariables.java
+++ b/src/main/java/org/shedlang/compiler/typechecker/DeclaredVariables.java
@@ -102,6 +102,9 @@ class DeclaredVariables {
                     VariableType.VAL,
                     val.source()
                 ));
+
+            case UntypedTargetNode.Ignore ignore ->
+                variables;
         };
     }
 }
diff --git a/src/main/java/org/shedlang/compiler/typechecker/TypeChecker.java b/src/main/java/org/shedlang/compiler/typechecker/TypeChecker.java
index 8f0d363a..2d426428 100644
--- a/src/main/java/org/shedlang/compiler/typechecker/TypeChecker.java
+++ b/src/main/java/org/shedlang/compiler/typechecker/TypeChecker.java
@@ -864,6 +864,10 @@ public class TypeChecker {
         TypeCheckerContext context
     ) {
         return switch (target) {
+            case UntypedTargetNode.Ignore ignore -> {
+                yield context.toResult(new TypedTargetNode.Ignore(target.source()));
+            }
+
             case UntypedTargetNode.Reassign reassign -> {
                 var targetVariableResult = runtimeLookup(
                     reassign.name(),
diff --git a/src/test/java/org/shedlang/compiler/ast/typed/Typed.java b/src/test/java/org/shedlang/compiler/ast/typed/Typed.java
index 0d48711c..42a46859 100644
--- a/src/test/java/org/shedlang/compiler/ast/typed/Typed.java
+++ b/src/test/java/org/shedlang/compiler/ast/typed/Typed.java
@@ -182,6 +182,10 @@ public class Typed {
         );
     }
 
+    public static TypedTargetNode targetIgnore() {
+        return new TypedTargetNode.Ignore(Sources.NULL);
+    }
+
     public static TypedTargetNode targetReassign(String name) {
         return new TypedTargetNode.Reassign(name, Sources.NULL);
     }
diff --git a/src/test/java/org/shedlang/compiler/ast/typed/TypedNodeMatchers.java b/src/test/java/org/shedlang/compiler/ast/typed/TypedNodeMatchers.java
index c8b0501d..52d91cdf 100644
--- a/src/test/java/org/shedlang/compiler/ast/typed/TypedNodeMatchers.java
+++ b/src/test/java/org/shedlang/compiler/ast/typed/TypedNodeMatchers.java
@@ -221,6 +221,10 @@ public class TypedNodeMatchers {
         );
     }
 
+    public static Matcher<TypedTargetNode> isTypedTargetIgnore() {
+        return instanceOf(TypedTargetNode.Ignore.class);
+    }
+
     public static Matcher<TypedTargetNode> isTypedTargetReassign(String name) {
         return instanceOf(
             TypedTargetNode.Reassign.class,
diff --git a/src/test/java/org/shedlang/compiler/ast/untyped/Untyped.java b/src/test/java/org/shedlang/compiler/ast/untyped/Untyped.java
index 36668a8b..a87b1c48 100644
--- a/src/test/java/org/shedlang/compiler/ast/untyped/Untyped.java
+++ b/src/test/java/org/shedlang/compiler/ast/untyped/Untyped.java
@@ -150,6 +150,10 @@ public class Untyped {
         );
     }
 
+    public static UntypedTargetNode targetIgnore() {
+        return new UntypedTargetNode.Ignore(Sources.NULL);
+    }
+
     public static UntypedTargetNode targetReassign(String name) {
         return new UntypedTargetNode.Reassign(name, Sources.NULL);
     }
diff --git a/src/test/java/org/shedlang/compiler/ast/untyped/UntypedNodeMatchers.java b/src/test/java/org/shedlang/compiler/ast/untyped/UntypedNodeMatchers.java
index 37d432e8..7d80b4ab 100644
--- a/src/test/java/org/shedlang/compiler/ast/untyped/UntypedNodeMatchers.java
+++ b/src/test/java/org/shedlang/compiler/ast/untyped/UntypedNodeMatchers.java
@@ -190,6 +190,10 @@ public class UntypedNodeMatchers {
         return isUntypedBinaryOperationNode(UntypedBinaryOperator.SUB, left, right);
     }
 
+    public static Matcher<UntypedTargetNode> isUntypedTargetIgnoreNode() {
+        return instanceOf(UntypedTargetNode.Ignore.class);
+    }
+
     public static Matcher<UntypedTargetNode> isUntypedTargetReassignNode(String name) {
         return instanceOf(
             UntypedTargetNode.Reassign.class,
diff --git a/src/test/java/org/shedlang/compiler/parser/ParserAssignTests.java b/src/test/java/org/shedlang/compiler/parser/ParserAssignTests.java
index 9c29d328..7dc07722 100644
--- a/src/test/java/org/shedlang/compiler/parser/ParserAssignTests.java
+++ b/src/test/java/org/shedlang/compiler/parser/ParserAssignTests.java
@@ -34,4 +34,16 @@ public class ParserAssignTests {
             isUntypedReferenceNode("z")
         ));
     }
+
+    @Test
+    public void canParseIgnoreTarget() {
+        var source = "_ = x;";
+
+        var node = parseString(source, Parser::parseFunctionStatement);
+
+        assertThat(node, isUntypedAssignNode(
+            isUntypedTargetIgnoreNode(),
+            isUntypedReferenceNode("x")
+        ));
+    }
 }
diff --git a/src/test/java/org/shedlang/compiler/stackir/StackCompilerAssignTests.java b/src/test/java/org/shedlang/compiler/stackir/StackCompilerAssignTests.java
index b717648a..7c49af0a 100644
--- a/src/test/java/org/shedlang/compiler/stackir/StackCompilerAssignTests.java
+++ b/src/test/java/org/shedlang/compiler/stackir/StackCompilerAssignTests.java
@@ -11,6 +11,23 @@ import static org.zwobble.precisely.Matchers.isSequence;
 
 public class StackCompilerAssignTests {
     @Test
+    public void targetIgnoreIsCompiledToDiscard() {
+        var node = Typed.assign(
+            Typed.targetIgnore(),
+            Typed.reference("x")
+        );
+        var context = StackCompiler.Context.EMPTY
+            .addShedVariable("x");
+
+        var result = StackCompiler.compileFunctionStatement(node, context);
+
+        assertThat(result.instructions(), isSequence(
+            isLocalLoadInstruction("x__0"),
+            isDiscardInstruction()
+        ));
+    }
+
+    @Test
     public void whenTargetIsReassignThenExpressionIsEvaluatedAndStoredAsVar() {
         var node = Typed.assign(
             Typed.targetReassign("x"),
diff --git a/src/test/java/org/shedlang/compiler/typechecker/TypeCheckerAssignTests.java b/src/test/java/org/shedlang/compiler/typechecker/TypeCheckerAssignTests.java
index eb2a2db1..9bd07ffd 100644
--- a/src/test/java/org/shedlang/compiler/typechecker/TypeCheckerAssignTests.java
+++ b/src/test/java/org/shedlang/compiler/typechecker/TypeCheckerAssignTests.java
@@ -24,6 +24,22 @@ import static org.zwobble.precisely.Matchers.*;
 
 public class TypeCheckerAssignTests {
     @Test
+    public void canTypeCheckTargetIgnore() {
+        var untyped = Untyped.assign(Untyped.targetIgnore(), Untyped.stringLiteral("hello"));
+        var context = TypeCheckerContext.EMPTY
+            .enterNamespace(P.map(
+                Map.entry("x", Variable.defined(Types.STRING, VariableType.VAR))
+            ));
+
+        var typed = typeCheckFunctionStatement(untyped, context);
+
+        assertThat(typed.value(), isTypedAssignNode(
+            isTypedTargetIgnore(),
+            isTypedStringLiteralNode("hello")
+        ));
+    }
+
+    @Test
     public void canTypeCheckReassign() {
         var untyped = Untyped.assign(Untyped.targetReassign("x"), Untyped.stringLiteral("hello"));
         var context = TypeCheckerContext.EMPTY