UnificationStep + code style fixes

src/main/java/edu/kit/typicalc/model/TypeInferenceResult.java

@@ -3,6 +3,7 @@ package edu.kit.typicalc.model;
 import edu.kit.typicalc.model.type.Type;
 import edu.kit.typicalc.model.type.TypeVariable;
 
+import java.util.Collections;
 import java.util.List;
 
 /**
@@ -31,7 +32,7 @@ public class TypeInferenceResult {
      * @return the most general unifier, null if there is no valid mgu
      */
     protected List<Substitution> getMGU() {
-        return null;
+        return Collections.emptyList();
         // TODO
     }
 

src/main/java/edu/kit/typicalc/model/UnificationStep.java

@@ -1,4 +1,45 @@
 package edu.kit.typicalc.model;
 
+import edu.kit.typicalc.util.Result;
+
+import java.util.List;
+
+/**
+ * Models one step of the unification algorithm with a list of constraints and a list of substitutions.
+ * These lists contain all the constraints or substitutions that are present in the unification after this step.
+ * When detected that this unification step leads to a contradiction or an infinite type,
+ * it contains a {@link UnificationError} instead of a list of substitutions.
+ */
 public class UnificationStep {
+    private final Result<List<Substitution>, UnificationError> substitutions;
+    private final List<Constraint> constraints;
+
+    /**
+     * Initializes a new {@link UnificationStep} with the given lists of constraints and substitutions.
+     * 	When detected that this unification step leads to a contradiction or an infinite type,
+     * 	it should be passed a {@link UnificationError} instead of a list of substitutions.
+     * @param substitutions list of substitutions, or an error
+     * @param constraints the list of all constraints of the unification (in the state resulting from this step)
+     */
+    protected UnificationStep(Result<List<Substitution>, UnificationError> substitutions,
+                              List<Constraint> constraints) {
+        this.substitutions = substitutions;
+        this.constraints = constraints;
+    }
+
+    /**
+     * Returns as a {@link Result} either the list of all substitutions
+     * of the unification (in the state resulting from this step).
+     * Or, in case of a detected contradiction or infinite type, a {@link UnificationError}.
+     */
+    public Result<List<Substitution>, UnificationError> getSubstitutions() {
+        return substitutions;
+    }
+
+    /**
+     * @return a list of all resulting constraints
+     */
+    public List<Constraint> getConstraints() {
+        return constraints;
+    }
 }

src/main/java/edu/kit/typicalc/model/parser/LambdaLexer.java

@@ -28,10 +28,6 @@ public class LambdaLexer {
      */
     public LambdaLexer(String term) {
         this.term = term;
-        tokenize();
-    }
-
-    private void tokenize() {
         Deque<Token> tokens = new ArrayDeque<>();
         while (true) {
             Result<Token, ParseError> token = parseNextToken();

src/main/java/edu/kit/typicalc/model/parser/ParseError.java

@@ -17,7 +17,7 @@ public enum ParseError {
      */
     UNEXPECTED_CHARACTER;
 
-    private Token cause;
+    private Token cause = new Token(Token.TokenType.EOF, "", -1);
 
     public ParseError withToken(Token cause) {
         this.cause = cause;

src/main/java/edu/kit/typicalc/model/step/InferenceStep.java

@@ -15,8 +15,8 @@ import java.util.Objects;
  * The subclasses vary in the premise(s) they contain.
  */
 public abstract class InferenceStep {
-    private Conclusion conclusion;
+    private final Conclusion conclusion;
-    private Constraint constraint;
+    private final Constraint constraint;
 
     /**
      * Initializes a new InferenceStep with the given values.

src/main/java/edu/kit/typicalc/model/step/LetStep.java

@@ -4,6 +4,8 @@ import edu.kit.typicalc.model.Conclusion;
 import edu.kit.typicalc.model.Constraint;
 import edu.kit.typicalc.model.TypeInfererLet;
 
+import java.util.Objects;
+
 /**
  * Models one step of the inference tree where the let rule is applied. A let step contains an
  * additional instance of a type inferer that is responisble for the „sub-inference“ that takes
@@ -46,4 +48,24 @@ public abstract class LetStep extends InferenceStep {
     public TypeInfererLet getTypeInferer() {
         return typeInferer;
     }
+
+    @Override
+    public boolean equals(Object o) {
+        if (this == o) {
+            return true;
+        }
+        if (o == null || getClass() != o.getClass()) {
+            return false;
+        }
+        if (!super.equals(o)) {
+            return false;
+        }
+        LetStep letStep = (LetStep) o;
+        return premise.equals(letStep.premise) && typeInferer.equals(letStep.typeInferer);
+    }
+
+    @Override
+    public int hashCode() {
+        return Objects.hash(super.hashCode(), premise, typeInferer);
+    }
 }

src/main/java/edu/kit/typicalc/model/type/UnificationUtil.java

@@ -13,10 +13,7 @@ import java.util.Optional;
  * Utility class to avoid unification logic duplication in type methods.
  */
 final class UnificationUtil {
-    // TODO: document class? implementation detail?
+    private UnificationUtil() { }
-    private UnificationUtil() {
-
-    }
 
     static Result<UnificationActions, UnificationError> functionFunction(FunctionType a, FunctionType b) {
         return new Result<>(new UnificationActions(List.of(