25 files changed, 311 insertions, 103 deletions

diff --git a/checker/build.gradle.kts b/checker/build.gradle.kts
new file mode 100644
index 0000000..08e9b33
--- /dev/null
+++ b/checker/build.gradle.kts
@@ -0,0 +1,30 @@
+plugins {
+    java
+    id("io.freefair.lombok") version "8.6"
+}
+
+val slf4jVersion = "2.0.13"
+
+repositories {
+    mavenCentral()
+}
+
+dependencies {
+    implementation("org.slf4j:slf4j-api:$slf4jVersion")
+
+    implementation(project(":ast"))
+    implementation(project(":core"))
+    implementation(project(":utils"))
+}
+
+java {
+    sourceCompatibility = JavaVersion.VERSION_22
+    targetCompatibility = JavaVersion.VERSION_22
+    toolchain {
+        languageVersion = JavaLanguageVersion.of(22)
+    }
+}
+
+tasks.withType<JavaCompile> {
+    options.compilerArgs.add("--enable-preview")
+}
\ No newline at end of file
diff --git a/checker/src/main/java/lv/enes/orang/checker/Checker.java b/checker/src/main/java/lv/enes/orang/checker/Checker.java
new file mode 100644
index 0000000..446d3f1
--- /dev/null
+++ b/checker/src/main/java/lv/enes/orang/checker/Checker.java
@@ -0,0 +1,146 @@
+package lv.enes.orang.checker;
+
+import lv.enes.orang.ast.*;
+import lv.enes.orang.core.ImmutableScope;
+import lv.enes.orang.core.Scope;
+
+import java.util.HashMap;
+import java.util.Map;
+
+public class Checker implements ExpressionVisitor<Void, CheckerException>, StatementVisitor<Checker, CheckerException> {
+        private final Scope<Boolean> definitions;
+
+        public static <E> Checker of(Map<String, E> builtins) {
+                var boolMap = builtins.keySet()
+                                .stream()
+                                .<Map<String, Boolean>>collect(
+                                                HashMap::new,
+                                                (map, elem) -> map.put(elem, true),
+                                                Map::putAll
+                                );
+                return new Checker(ImmutableScope.of(boolMap));
+        }
+
+        private Checker(Scope<Boolean> definitions) {
+                this.definitions = definitions;
+        }
+
+        @Override
+        public Void visitArray(ArrayExpression array) throws CheckerException {
+                for (var expr : array.items()) {
+                        visit(expr);
+                }
+                return null;
+        }
+
+        @Override
+        public Void visitBoolean(BooleanLiteral expr) {
+                // Always ok
+                return null;
+        }
+
+        @Override
+        public Void visitBinaryExpression(BinaryExpression expr) throws CheckerException {
+                visit(expr.lhs());
+                visit(expr.rhs());
+                return null;
+        }
+
+        @Override
+        public Void visitCallExpression(CallExpression expr) throws CheckerException {
+                visit(expr.callee());
+                visit(expr.arg());
+                return null;
+        }
+
+        @Override
+        public Checker visitDefinition(Definition def) throws CheckerException {
+                if (definitions.hasDefinition(def.name())) {
+                        throw new CheckerException(STR."Top-level definition '\{def.name()}' redefined!");
+                }
+                return new Checker(ImmutableScope.of(definitions, def.name(), true));
+        }
+
+        @Override
+        public Void visitDoExpression(DoExpression expr) throws CheckerException {
+                for (var child : expr.body()) {
+                        visit(child);
+                }
+                return null;
+        }
+
+        @Override
+        public Checker visitExpression(ExpressionStatement expr) throws CheckerException {
+                visit(expr.expr());
+                return this;
+        }
+
+        @Override
+        public Void visitFnExpression(FnExpression expr) throws CheckerException {
+                var args = new HashMap<String, Boolean>();
+                for (var arg : expr.args()) {
+                        args.put(arg.name, true);
+                }
+                new Checker(ImmutableScope.of(definitions, args)).visit(expr.body());
+                return null;
+        }
+
+        @Override
+        public Void visitIfElseExpression(IfElseExpression expr) throws CheckerException {
+                visit(expr.condition());
+                visit(expr.trueBranch());
+                visit(expr.falseBranch());
+                return null;
+        }
+
+        @Override
+        public Void visitIntLiteral(IntLiteral expr) {
+                // Always ok
+                return null;
+        }
+
+        @Override
+        public Void visitLetInExpression(LetInExpression expr) throws CheckerException {
+                var locals = new HashMap<String, Boolean>();
+                for (var local : expr.bindings()) {
+                        locals.put(local.name(), true);
+                }
+                new Checker(ImmutableScope.of(definitions, locals)).visit(expr.body());
+                return null;
+        }
+
+        @Override
+        public Checker visitProgram(Program program) throws CheckerException {
+                var checker = this;
+                for (var stmt : program.statements()) {
+                        checker = checker.visit(stmt);
+                }
+                return checker;
+        }
+
+        @Override
+        public Void visitStringLiteral(StringLiteral expr) {
+                // Always ok
+                return null;
+        }
+
+        @Override
+        public Void visitUnaryExpression(UnaryExpression expr) throws CheckerException {
+                visit(expr.child());
+                return null;
+        }
+
+        @Override
+        public Void visitVariable(VariableExpression expr) throws CheckerException {
+                if (!definitions.hasDefinition(expr.name())) {
+                        throw new CheckerException(STR."Variable named '\{expr.name()}' not defined!");
+                }
+                return null;
+        }
+
+        @Override
+        public Void visitVoidExpression() {
+                // Always ok
+                return null;
+        }
+}
diff --git a/checker/src/main/java/lv/enes/orang/checker/CheckerException.java b/checker/src/main/java/lv/enes/orang/checker/CheckerException.java
new file mode 100644
index 0000000..9354f27
--- /dev/null
+++ b/checker/src/main/java/lv/enes/orang/checker/CheckerException.java
@@ -0,0 +1,9 @@
+package lv.enes.orang.checker;
+
+import lv.enes.orang.core.OrangException;
+
+public class CheckerException extends OrangException {
+        public CheckerException(String message) {
+                super(message);
+        }
+}
diff --git a/checker/src/main/java/module-info.java b/checker/src/main/java/module-info.java
new file mode 100644
index 0000000..6e5fa47
--- /dev/null
+++ b/checker/src/main/java/module-info.java
@@ -0,0 +1,9 @@
+module lv.enes.orang.checker {
+        exports lv.enes.orang.checker;
+
+        requires lv.enes.orang.ast;
+        requires lv.enes.orang.core;
+        requires lv.enes.orang.utils;
+
+        requires static lombok;
+}
\ No newline at end of file
diff --git a/core/src/main/java/lv/enes/orang/core/ImmutableScope.java b/core/src/main/java/lv/enes/orang/core/ImmutableScope.java
new file mode 100644
index 0000000..5120b08
--- /dev/null
+++ b/core/src/main/java/lv/enes/orang/core/ImmutableScope.java
@@ -0,0 +1,47 @@
+package lv.enes.orang.core;
+
+import java.util.HashMap;
+import java.util.Map;
+
+public class ImmutableScope<E> extends Scope<E> {
+        public static <E> ImmutableScope<E> of(Map<String, E> builtins) {
+                return new ImmutableScope<>(null, new HashMap<>(builtins));
+        }
+
+        public static <E> ImmutableScope<E> of(Scope<E> parent) {
+                if (parent instanceof ImmutableScope<E> imm) {
+                        return imm;
+                }
+                return ImmutableScope.of(parent, Map.of());
+        }
+
+        public static <E> ImmutableScope<E> of(Scope<E> parent, String key, E value) {
+                return ImmutableScope.of(parent, Map.of(key, value));
+        }
+
+        public static <E> ImmutableScope<E> of(Scope<E> parent, Map<String, E> definitions) {
+                return new ImmutableScope<>(parent, definitions).maybeFlattened();
+        }
+
+        protected ImmutableScope(Scope<E> parent, Map<String, E> definitions) {
+                super(parent, Map.copyOf(definitions));
+        }
+
+        public ImmutableScope<E> maybeFlattened() {
+                if (depth > MAX_DEPTH) {
+                        return flattened();
+                }
+                return this;
+        }
+
+        private ImmutableScope<E> flattened() {
+                if (parent instanceof ImmutableScope<E> immParent) {
+                        var flatParent = immParent.flattened();
+                        var newDefs = new HashMap<>(flatParent.definitions);
+                        newDefs.putAll(definitions);
+                        return new ImmutableScope<>(flatParent.parent, newDefs);
+                }
+
+                return this;
+        }
+}
diff --git a/core/src/main/java/lv/enes/orang/core/MutableScope.java b/core/src/main/java/lv/enes/orang/core/MutableScope.java
new file mode 100644
index 0000000..8d8b455
--- /dev/null
+++ b/core/src/main/java/lv/enes/orang/core/MutableScope.java
@@ -0,0 +1,26 @@
+package lv.enes.orang.core;
+
+import java.util.HashMap;
+import java.util.Map;
+
+public class MutableScope<E> extends Scope<E> {
+        public static <E> MutableScope<E> of(Map<String, E> builtins) {
+                return new MutableScope<>(null, new HashMap<>(builtins));
+        }
+
+        public static <E> MutableScope<E> of(Scope<E> parent) {
+                return new MutableScope<>(parent, Map.of());
+        }
+
+        public static <E> MutableScope<E> of(Scope<E> parent, String name, E value) {
+                return new MutableScope<>(parent, Map.of(name, value));
+        }
+
+        protected MutableScope(Scope<E> parent, Map<String, E> definitions) {
+                super(parent, new HashMap<>(definitions));
+        }
+
+        public void setDefinition(String key, E value) {
+                definitions.put(key, value);
+        }
+}
diff --git a/orang/src/main/java/lv/enes/orang/OrangRuntimeException.java b/core/src/main/java/lv/enes/orang/core/OrangRuntimeException.java
index 8a7d575..6ce5f70 100644
--- a/orang/src/main/java/lv/enes/orang/OrangRuntimeException.java
+++ b/core/src/main/java/lv/enes/orang/core/OrangRuntimeException.java
@@ -1,6 +1,4 @@
-package lv.enes.orang;
-
-import lv.enes.orang.core.OrangException;
+package lv.enes.orang.core;
 
 public class OrangRuntimeException extends OrangException {
         public OrangRuntimeException(String message) {
diff --git a/orang/src/main/java/lv/enes/orang/Scope.java b/core/src/main/java/lv/enes/orang/core/Scope.java
index 7fe57c7..0f4d23a 100644
--- a/orang/src/main/java/lv/enes/orang/Scope.java
+++ b/core/src/main/java/lv/enes/orang/core/Scope.java
@@ -1,28 +1,21 @@
-package lv.enes.orang;
+package lv.enes.orang.core;
 
-import lv.enes.orang.value.Value;
-
-import java.util.HashMap;
 import java.util.Map;
 
-public abstract class Scope {
+public abstract class Scope<E> {
         public static final int MAX_DEPTH = 4;
 
-        protected final Scope parent;
-        protected final Map<String, Value> definitions;
+        protected final Scope<E> parent;
+        protected final Map<String, E> definitions;
         protected final int depth;
 
-        public static MutableScope topLevel() {
-                return new MutableScope(null, new HashMap<>(Builtins.BUILTINS));
-        }
-
-        protected Scope(Scope parent, Map<String, Value> definitions) {
+        protected Scope(Scope<E> parent, Map<String, E> definitions) {
                 this.parent = parent;
                 this.definitions = definitions;
                 this.depth = parent == null ? 0 : parent.depth + 1;
         }
 
-        public Value getDefinition(String name) throws OrangRuntimeException {
+        public E getDefinition(String name) throws OrangRuntimeException {
                 if (definitions.containsKey(name)) {
                         return definitions.get(name);
                 } else if (parent != null) {
@@ -31,4 +24,8 @@ public abstract class Scope {
                         throw new OrangRuntimeException(STR."Value named \{name} is not defined!");
                 }
         }
+
+        public boolean hasDefinition(String name) {
+                return definitions.containsKey(name) || parent != null && parent.hasDefinition(name);
+        }
 }
diff --git a/orang/build.gradle.kts b/orang/build.gradle.kts
index ef78ded..ea8fbe0 100644
--- a/orang/build.gradle.kts
+++ b/orang/build.gradle.kts
@@ -23,6 +23,7 @@ dependencies {
     implementation("org.slf4j:slf4j-simple:$slf4jVersion")
 
     implementation(project(":ast"))
+    implementation(project(":checker"))
     implementation(project(":core"))
     implementation(project(":parser"))
     implementation(project(":utils"))
diff --git a/orang/src/main/java/lv/enes/orang/Builtins.java b/orang/src/main/java/lv/enes/orang/Builtins.java
index de296ac..a83f4c5 100644
--- a/orang/src/main/java/lv/enes/orang/Builtins.java
+++ b/orang/src/main/java/lv/enes/orang/Builtins.java
@@ -1,6 +1,7 @@
 package lv.enes.orang;
 
 import lombok.extern.slf4j.Slf4j;
+import lv.enes.orang.core.OrangRuntimeException;
 import lv.enes.orang.value.*;
 
 import java.io.IOException;
diff --git a/orang/src/main/java/lv/enes/orang/Evaluator.java b/orang/src/main/java/lv/enes/orang/Evaluator.java
index 5d09508..8930f5c 100644
--- a/orang/src/main/java/lv/enes/orang/Evaluator.java
+++ b/orang/src/main/java/lv/enes/orang/Evaluator.java
@@ -1,17 +1,20 @@
 package lv.enes.orang;
 
 import lv.enes.orang.ast.*;
+import lv.enes.orang.core.MutableScope;
+import lv.enes.orang.core.OrangRuntimeException;
+import lv.enes.orang.core.Scope;
 import lv.enes.orang.value.*;
 
 import java.util.ArrayList;
 import java.util.Collections;
 
-public record Evaluator(Scope scope, Value lastResult) implements ExpressionVisitor<Value, OrangRuntimeException>, StatementVisitor<Evaluator, OrangRuntimeException> {
+public record Evaluator(Scope<Value> scope, Value lastResult) implements ExpressionVisitor<Value, OrangRuntimeException>, StatementVisitor<Evaluator, OrangRuntimeException> {
         public Evaluator() {
-                this(Scope.topLevel());
+                this(MutableScope.of(Builtins.BUILTINS));
         }
 
-        public Evaluator(Scope scope) {
+        public Evaluator(Scope<Value> scope) {
                 this(scope, Undefined.INSTANCE);
         }
 
diff --git a/orang/src/main/java/lv/enes/orang/ImmutableScope.java b/orang/src/main/java/lv/enes/orang/ImmutableScope.java
deleted file mode 100644
index 9cb6138..0000000
--- a/orang/src/main/java/lv/enes/orang/ImmutableScope.java
+++ /dev/null
@@ -1,45 +0,0 @@
-package lv.enes.orang;
-
-import lv.enes.orang.value.Value;
-
-import java.util.HashMap;
-import java.util.Map;
-
-public class ImmutableScope extends Scope {
-        public static ImmutableScope of(Scope parent) {
-                if (parent instanceof ImmutableScope imm) {
-                        return imm;
-                }
-                return ImmutableScope.of(parent, Map.of());
-        }
-
-        public static ImmutableScope of(Scope parent, String key, Value value) {
-                return ImmutableScope.of(parent, Map.of(key, value));
-        }
-
-        public static ImmutableScope of(Scope parent, Map<String, Value> definitions) {
-                return new ImmutableScope(parent, definitions).maybeFlattened();
-        }
-
-        protected ImmutableScope(Scope parent, Map<String, Value> definitions) {
-                super(parent, Map.copyOf(definitions));
-        }
-
-        public ImmutableScope maybeFlattened() {
-                if (depth > MAX_DEPTH) {
-                        return flattened();
-                }
-                return this;
-        }
-
-        private ImmutableScope flattened() {
-                if (parent instanceof ImmutableScope immParent) {
-                        var flatParent = immParent.flattened();
-                        var newDefs = new HashMap<>(flatParent.definitions);
-                        newDefs.putAll(definitions);
-                        return new ImmutableScope(flatParent.parent, newDefs);
-                }
-
-                return this;
-        }
-}
diff --git a/orang/src/main/java/lv/enes/orang/Main.java b/orang/src/main/java/lv/enes/orang/Main.java
index 7175dac..9a061fe 100644
--- a/orang/src/main/java/lv/enes/orang/Main.java
+++ b/orang/src/main/java/lv/enes/orang/Main.java
@@ -1,5 +1,6 @@
 package lv.enes.orang;
 
+import lv.enes.orang.checker.Checker;
 import lv.enes.orang.core.OrangException;
 import lv.enes.orang.parser.Parser;
 
@@ -17,11 +18,13 @@ public class Main {
         }
 
         private static void repl() throws IOException {
+                var checker = Checker.of(Builtins.BUILTINS);
                 var evaluator = new Evaluator();
 
                 try (var stream = Main.class.getResourceAsStream("prelude.orang")) {
                         var prog = Parser.parseProgram(stream);
-                        evaluator = evaluator.visitProgram(prog);
+                        checker = checker.visit(prog);
+                        evaluator = evaluator.visit(prog);
                 } catch (OrangException ex) {
                         STDOUT.println(STR."While evaluating prelude: \{ex}");
                         throw new RuntimeException(ex);
@@ -51,7 +54,8 @@ public class Main {
                                                 var filename = line.substring(2).trim();
                                                 try (var reader = new FileReader((filename))) {
                                                         var prog = Parser.parseProgram(reader);
-                                                        evaluator = evaluator.visitProgram(prog);
+                                                        checker = checker.visit(prog);
+                                                        evaluator = evaluator.visit(prog);
                                                 } catch (IOException | OrangException ex) {
                                                         STDOUT.println(ex);
                                                 }
@@ -67,7 +71,8 @@ public class Main {
 
                         try {
                                 var prog = Parser.parseReplProgram(line);
-                                evaluator = evaluator.visitProgram(prog);
+                                checker = checker.visit(prog);
+                                evaluator = evaluator.visit(prog);
                                 if (evaluator.lastResult() != null) {
                                         STDOUT.print("-> ");
                                         STDOUT.println(evaluator.lastResult().stringify());
diff --git a/orang/src/main/java/lv/enes/orang/MutableScope.java b/orang/src/main/java/lv/enes/orang/MutableScope.java
deleted file mode 100644
index 8d900a4..0000000
--- a/orang/src/main/java/lv/enes/orang/MutableScope.java
+++ /dev/null
@@ -1,24 +0,0 @@
-package lv.enes.orang;
-
-import lv.enes.orang.value.Value;
-
-import java.util.HashMap;
-import java.util.Map;
-
-public class MutableScope extends Scope {
-        public static MutableScope of(Scope parent) {
-                return new MutableScope(parent, Map.of());
-        }
-
-        public static MutableScope of(Scope parent, String name, Value value) {
-                return new MutableScope(parent, Map.of(name, value));
-        }
-
-        protected MutableScope(Scope parent, Map<String, Value> definitions) {
-                super(parent, new HashMap<>(definitions));
-        }
-
-        public void setDefinition(String key, Value value) {
-                definitions.put(key, value);
-        }
-}
diff --git a/orang/src/main/java/lv/enes/orang/value/Array.java b/orang/src/main/java/lv/enes/orang/value/Array.java
index c3b645d..33e1516 100644
--- a/orang/src/main/java/lv/enes/orang/value/Array.java
+++ b/orang/src/main/java/lv/enes/orang/value/Array.java
@@ -1,6 +1,6 @@
 package lv.enes.orang.value;
 
-import lv.enes.orang.OrangRuntimeException;
+import lv.enes.orang.core.OrangRuntimeException;
 
 import java.util.ArrayList;
 import java.util.Collections;
diff --git a/orang/src/main/java/lv/enes/orang/value/BuiltinFunction.java b/orang/src/main/java/lv/enes/orang/value/BuiltinFunction.java
index 92ccc2e..1a61cfb 100644
--- a/orang/src/main/java/lv/enes/orang/value/BuiltinFunction.java
+++ b/orang/src/main/java/lv/enes/orang/value/BuiltinFunction.java
@@ -1,6 +1,6 @@
 package lv.enes.orang.value;
 
-import lv.enes.orang.OrangRuntimeException;
+import lv.enes.orang.core.OrangRuntimeException;
 
 import java.util.List;
 
diff --git a/orang/src/main/java/lv/enes/orang/value/Function.java b/orang/src/main/java/lv/enes/orang/value/Function.java
index 6e3c83a..d0500b9 100644
--- a/orang/src/main/java/lv/enes/orang/value/Function.java
+++ b/orang/src/main/java/lv/enes/orang/value/Function.java
@@ -1,11 +1,12 @@
 package lv.enes.orang.value;
 
-import lv.enes.orang.*;
 import lv.enes.orang.ast.ArgSpec;
 import lv.enes.orang.ast.Expression;
+import lv.enes.orang.core.OrangRuntimeException;
+import lv.enes.orang.core.Scope;
 import lv.enes.orang.utils.NonEmptyList;
 
-public record Function(Scope scope, NonEmptyList<ArgSpec> args, Expression body) implements Value {
+public record Function(Scope<Value> scope, NonEmptyList<ArgSpec> args, Expression body) implements Value {
         @Override
         public String typeName() {
                 return "Function";
diff --git a/orang/src/main/java/lv/enes/orang/value/OrangBoolean.java b/orang/src/main/java/lv/enes/orang/value/OrangBoolean.java
index 8a4776c..59aa9b7 100644
--- a/orang/src/main/java/lv/enes/orang/value/OrangBoolean.java
+++ b/orang/src/main/java/lv/enes/orang/value/OrangBoolean.java
@@ -1,7 +1,7 @@
 package lv.enes.orang.value;
 
 import lombok.EqualsAndHashCode;
-import lv.enes.orang.OrangRuntimeException;
+import lv.enes.orang.core.OrangRuntimeException;
 
 @EqualsAndHashCode
 public final class OrangBoolean implements Value {
diff --git a/orang/src/main/java/lv/enes/orang/value/OrangInteger.java b/orang/src/main/java/lv/enes/orang/value/OrangInteger.java
index 9b8d505..63a44da 100644
--- a/orang/src/main/java/lv/enes/orang/value/OrangInteger.java
+++ b/orang/src/main/java/lv/enes/orang/value/OrangInteger.java
@@ -1,6 +1,6 @@
 package lv.enes.orang.value;
 
-import lv.enes.orang.OrangRuntimeException;
+import lv.enes.orang.core.OrangRuntimeException;
 
 public record OrangInteger(int value) implements Value {
         @Override
diff --git a/orang/src/main/java/lv/enes/orang/value/OrangString.java b/orang/src/main/java/lv/enes/orang/value/OrangString.java
index c03f7ac..b4ede1f 100644
--- a/orang/src/main/java/lv/enes/orang/value/OrangString.java
+++ b/orang/src/main/java/lv/enes/orang/value/OrangString.java
@@ -1,6 +1,6 @@
 package lv.enes.orang.value;
 
-import lv.enes.orang.OrangRuntimeException;
+import lv.enes.orang.core.OrangRuntimeException;
 
 public record OrangString(String value) implements Value {
         @Override
diff --git a/orang/src/main/java/lv/enes/orang/value/PartialBuiltinFunction.java b/orang/src/main/java/lv/enes/orang/value/PartialBuiltinFunction.java
index 37278a4..c1be22a 100644
--- a/orang/src/main/java/lv/enes/orang/value/PartialBuiltinFunction.java
+++ b/orang/src/main/java/lv/enes/orang/value/PartialBuiltinFunction.java
@@ -1,6 +1,6 @@
 package lv.enes.orang.value;
 
-import lv.enes.orang.OrangRuntimeException;
+import lv.enes.orang.core.OrangRuntimeException;
 
 import java.util.ArrayList;
 import java.util.Collections;
diff --git a/orang/src/main/java/lv/enes/orang/value/PartialFunction.java b/orang/src/main/java/lv/enes/orang/value/PartialFunction.java
index cee0cd4..e73ba0c 100644
--- a/orang/src/main/java/lv/enes/orang/value/PartialFunction.java
+++ b/orang/src/main/java/lv/enes/orang/value/PartialFunction.java
@@ -3,10 +3,13 @@ package lv.enes.orang.value;
 import lv.enes.orang.*;
 import lv.enes.orang.ast.ArgSpec;
 import lv.enes.orang.ast.Expression;
+import lv.enes.orang.core.MutableScope;
+import lv.enes.orang.core.OrangRuntimeException;
+import lv.enes.orang.core.Scope;
 import lv.enes.orang.utils.NonEmptyList;
 
-public record PartialFunction(Scope scope, NonEmptyList<ArgSpec> remainingArgs, Expression body) implements Value {
-        public static Value of(Scope scope, NonEmptyList<ArgSpec> remainingArgs, Expression body, Value param) throws OrangRuntimeException {
+public record PartialFunction(Scope<Value> scope, NonEmptyList<ArgSpec> remainingArgs, Expression body) implements Value {
+        public static Value of(Scope<Value> scope, NonEmptyList<ArgSpec> remainingArgs, Expression body, Value param) throws OrangRuntimeException {
                 var spec = remainingArgs.getFirst();
                 var newScope = MutableScope.of(scope);
                 switch (spec.type) {
diff --git a/orang/src/main/java/lv/enes/orang/value/Value.java b/orang/src/main/java/lv/enes/orang/value/Value.java
index 0816dc2..07e6848 100644
--- a/orang/src/main/java/lv/enes/orang/value/Value.java
+++ b/orang/src/main/java/lv/enes/orang/value/Value.java
@@ -1,6 +1,6 @@
 package lv.enes.orang.value;
 
-import lv.enes.orang.OrangRuntimeException;
+import lv.enes.orang.core.OrangRuntimeException;
 
 public sealed interface Value
                 permits Array, BuiltinFunction, Function, Nothing, OrangBoolean, OrangInteger, OrangString, PartialBuiltinFunction, PartialFunction, Undefined {
diff --git a/orang/src/main/java/module-info.java b/orang/src/main/java/module-info.java
index 353d0dd..5b7b601 100644
--- a/orang/src/main/java/module-info.java
+++ b/orang/src/main/java/module-info.java
@@ -3,6 +3,7 @@ module lv.enes.orang {
         exports lv.enes.orang.value;
 
         requires lv.enes.orang.ast;
+        requires lv.enes.orang.checker;
         requires lv.enes.orang.core;
         requires lv.enes.orang.parser;
         requires lv.enes.orang.utils;
diff --git a/settings.gradle.kts b/settings.gradle.kts
index 391d33b..a64dfa6 100644
--- a/settings.gradle.kts
+++ b/settings.gradle.kts
@@ -1,3 +1,3 @@
 rootProject.name = "orang"
 
-include("ast", "core", "lexer", "orang", "parser", "utils")
\ No newline at end of file
+include("ast", "checker", "core", "lexer", "orang", "parser", "utils")
\ No newline at end of file