fix: add function must return on all paths check

src/main/wacc/ast.scala

@@ -1,7 +1,9 @@
 package wacc
 
+import parsley.errors.combinator._
 import parsley.generic._
 import cats.data.NonEmptyList
+import parsley.Parsley
 
 object ast {
   // Expressions
@@ -96,7 +98,18 @@ object ast {
       params: List[Param],
       body: NonEmptyList[Stmt]
   )
-  object FuncDecl extends ParserBridge4[Type, Ident, List[Param], NonEmptyList[Stmt], FuncDecl]
+  object FuncDecl extends ParserBridge4[Type, Ident, List[Param], NonEmptyList[Stmt], FuncDecl] {
+    override def apply(
+        x1: Parsley[Type],
+        x2: => Parsley[Ident],
+        x3: => Parsley[List[Param]],
+        x4: => Parsley[NonEmptyList[Stmt]]
+    ): Parsley[FuncDecl] =
+      super.apply(x1, x2, x3, x4).guardAgainst {
+        case FuncDecl(_, _, _, body) if !body.isReturning =>
+          Seq("Function must return on all paths")
+      }
+  }
 
   case class Param(paramType: Type, name: Ident)
   object Param extends ParserBridge2[Type, Ident, Param]
@@ -140,4 +153,14 @@ object ast {
   object Fst extends ParserBridge1[LValue, Fst]
   case class Snd(elem: LValue) extends PairElem
   object Snd extends ParserBridge1[LValue, Snd]
+
+  extension (stmts: NonEmptyList[Stmt]) {
+    def isReturning: Boolean = stmts.last match {
+      case Return(_) | Exit(_)       => true
+      case If(_, thenStmt, elseStmt) => thenStmt.isReturning && elseStmt.isReturning
+      case While(_, body)            => body.isReturning
+      case Block(body)               => body.isReturning
+      case _                         => false
+    }
+  }
 }