Save expected input type in ParseError

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

@@ -0,0 +1,11 @@
+package edu.kit.typicalc.model.parser;
+
+/**
+ * Attached to a {@link ParseError} to signify what kind of input is expected.
+ */
+public enum ExpectedInput {
+    /**
+     * Any kind of lambda term.
+     */
+    TERM
+}

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

@@ -217,7 +217,7 @@ public class LambdaParser {
             default:
                 error = expect(TokenType.LEFT_PARENTHESIS);
                 if (error.isPresent()) {
-                    return new Result<>(null, error.get());
+                    return new Result<>(null, error.get().expectedInput(ExpectedInput.TERM));
                 }
                 Result<LambdaTerm, ParseError> term = parseTerm(false);
                 error = expect(TokenType.RIGHT_PARENTHESIS);

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

@@ -29,6 +29,7 @@ public enum ParseError {
 
     private Optional<Token> cause = Optional.empty();
     private Optional<Collection<Token.TokenType>> needed = Optional.empty();
+    private Optional<ExpectedInput> expected = Optional.empty();
     private String term = "";
     private char wrongChar = '\0';
     private char correctChar = '\0';
@@ -44,6 +45,7 @@ public enum ParseError {
     public ParseError withToken(Token cause, String term) {
         this.cause = Optional.of(cause);
         this.term = term;
+        this.position = cause.getPos();
         return this;
     }
 
@@ -69,6 +71,22 @@ public enum ParseError {
         return this;
     }
 
+    /**
+     * Store which kind of input is expected. Clears expected tokens.
+     *
+     * @param input expected input
+     * @return this object
+     */
+    public ParseError expectedInput(ExpectedInput input) {
+        this.needed = Optional.empty();
+        this.expected = Optional.of(input);
+        return this;
+    }
+
+    public Optional<ExpectedInput> getExpectedInput() {
+        return this.expected;
+    }
+
     /**
      * Attach an expected character to this error.
      *

src/test/java/edu/kit/typicalc/model/parser/LambdaParserTest.java

@@ -184,6 +184,14 @@ class LambdaParserTest {
         assertEquals(new AppTerm(new AppTerm(new AbsTerm(X, X), new AbsTerm(X, X)), new AbsTerm(X, X)), term.unwrap());
     }
 
+    @Test
+    void usefulErrors() {
+        LambdaParser parser = new LambdaParser("λx..");
+        ParseError error = parser.parse().unwrapError();
+        assertEquals(ExpectedInput.TERM, error.getExpectedInput().get());
+        assertEquals(3, error.getPosition());
+    }
+
     @Test
     void equality() {
         EqualsVerifier.forClass(Token.class).usingGetClass().verify();