Add tests for toString (#273)

* Add tests for toString

* Make tests more independent

tests/src/test/scala/org/dhallj/tests/ToStringSuite.scala

@@ -1,11 +1,12 @@
 package org.dhallj.tests
 
-import munit.{ScalaCheckSuite, Slow}
-import org.dhallj.core.Expr
+import munit.{Ignore, ScalaCheckSuite, Slow}
+import org.dhallj.core.{Expr, Operator}
 import org.dhallj.parser.DhallParser
 import org.dhallj.testing.WellTypedExpr
 import org.dhallj.testing.instances._
 import org.scalacheck.Prop
+import scala.util.Try
 
 class ToStringSuite extends ScalaCheckSuite() {
   property("toString produces parseable code given well-typed values") {
@@ -16,6 +17,26 @@ class ToStringSuite extends ScalaCheckSuite() {
     Prop.forAll((expr: Expr) => DhallParser.parse(clue(expr.toString)) == expr)
   }
 
+  property("toString produces parseable code for sequence of operators") {
+    Prop.forAll { (ops: Vector[Operator]) =>
+      val x = Expr.makeIdentifier("x")
+      val expr1 = ops.take(16).foldLeft(x) { case (acc, op) => Expr.makeOperatorApplication(op, x, acc) }
+      val expr2 = ops.take(16).foldLeft(x) { case (acc, op) => Expr.makeOperatorApplication(op, acc, x) }
+      DhallParser.parse(clue(expr1.toString)) == expr1 && DhallParser.parse(clue(expr2.toString)) == expr2
+    }
+  }
+
+  // TODO: make this pass (e.g. seed rBdMB22PVU38DRrcK3rrh1kEBDitFj8dL1tbCloBmgM=)
+  property("toString doesn't introduce unnecessary parentheses".tag(Ignore)) {
+    Prop.forAll { (expr: Expr) =>
+      val asString = expr.toString
+
+      removeParentheses(asString).forall { smaller =>
+        !Try(DhallParser.parse(smaller)).toOption.exists(_ == expr)
+      }
+    }
+  }
+
   test("Unnormalized Prelude should round-trip through toString".tag(Slow)) {
     import org.dhallj.syntax._
 
@@ -25,4 +46,34 @@ class ToStringSuite extends ScalaCheckSuite() {
     assert(prelude.hash.sameElements(fromToString.hash))
     assert(prelude.diff(fromToString).isEmpty)
   }
+
+  private def closingIndex(input: String, startIndex: Int): Int = {
+    var i = startIndex + 1
+    var depth = 1
+
+    while (i < input.length && depth > 0) {
+      if (input.charAt(i) == ')') {
+        depth -= 1
+      } else if (input.charAt(i) == '(') {
+        depth += 1
+      }
+      i += 1
+    }
+
+    i - 1
+  }
+
+  // Assumes valid nesting
+  private def removeParentheses(input: String, first: Int = 0): List[String] = {
+    val nextIndex = input.indexOf("(", first)
+
+    if (nextIndex < 0) {
+      Nil
+    } else {
+      val end = closingIndex(input, nextIndex)
+      val next = input.take(nextIndex) + input.substring(nextIndex + 1, end) + input.substring(end + 1)
+
+      next :: removeParentheses(input, nextIndex + 1)
+    }
+  }
 }