Fix parser issue

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

@@ -19,7 +19,7 @@ import java.util.Set;
  *
  * @see LambdaTerm
  */
-public class LambdaParser { // TODO: document syntax above ^ ?
+public class LambdaParser {
     /**
      * lexer to translate a String into tokens
      */
@@ -152,10 +152,13 @@ public class LambdaParser { // TODO: document syntax above ^ ?
         if (left.isError()) {
             return left;
         }
-        while (ATOM_START_TOKENS.contains(token.getType()) || token.getType() == TokenType.LAMBDA) {
+        while (ATOM_START_TOKENS.contains(token.getType())
+                || token.getType() == TokenType.LAMBDA || token.getType() == TokenType.LET) {
             Result<LambdaTerm, ParseError> atom;
             if (token.getType() == TokenType.LAMBDA) {
                 atom = new Result<>(parseAbstraction());
+            } else if (token.getType() == TokenType.LET) {
+                atom = new Result<>(parseLet());
             } else {
                 atom = parseAtom();
             }

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

@@ -10,6 +10,7 @@ import static org.junit.jupiter.api.Assertions.assertEquals;
 
 class LambdaParserTest {
     private static final VarTerm X = new VarTerm("x");
+    private static final VarTerm Y = new VarTerm("y");
 
     @Test
     void varTerm() {
@@ -155,6 +156,23 @@ class LambdaParserTest {
         assertEquals(new AppTerm(new AbsTerm(X, X), new AbsTerm(X, X)), term.unwrap());
     }
 
+    @Test
+    void bugFoundByJohanna() {
+        // original term: (λx.λy.y (x y)) (λz. λa. z g a) let f = λx. let g = λy. y in g x in f 3
+        // reduced:
+        LambdaParser parser = new LambdaParser("(λx.x) let id = λy.y in id");
+        Result<LambdaTerm, ParseError> term = parser.parse();
+        if (term.isError()) {
+            System.err.println(term.unwrapError());
+            System.err.println(term.unwrapError().getCause());
+        }
+        assertEquals(new AppTerm(
+                new AbsTerm(X, X),
+                new LetTerm(new VarTerm("id"), new AbsTerm(Y, Y), new VarTerm("id"))
+                ),
+                term.unwrap());
+    }
+
     @Test
     void complicatedIdentity() {
         LambdaParser parser = new LambdaParser("(λx. x) (λx. x) λx. x");