Fix primitive string comparison on Chez (#792)

Found when debugging #561 -- string comparison was present on Chez, but
it never worked as strings are compared with operations that look like
`string<?`.
See section 7.4 here https://www.scheme.com/csug8/objects.html :)

I also added a tiny test to ensure that this doesn't confuse anybody
else down the line.

Author: jiribenes

effekt/jvm/src/test/scala/effekt/StdlibTests.scala

@@ -46,5 +46,8 @@ class StdlibLLVMTests extends StdlibTests {
 
     // Conditional jump or move depends on uninitialised value(s)
     examplesDir / "stdlib" / "io" / "filesystem" / "wordcount.effekt",
+
+    // String comparison using `<`, `<=`, `>`, `>=` is not implemented yet on LLVM
+    examplesDir / "stdlib" / "string" / "compare.effekt",
   )
 }

examples/stdlib/string/compare.check

@@ -0,0 +1,35 @@
+false
+true
+false
+true
+false
+true
+false
+true
+
+true
+true
+false
+false
+false
+false
+true
+true
+
+true
+true
+false
+false
+false
+false
+true
+true
+
+true
+true
+false
+false
+false
+false
+true
+true

examples/stdlib/string/compare.effekt

@@ -0,0 +1,46 @@
+def main() = {
+  val empty = ""
+  val a = "a"
+
+  println(empty < empty)
+  println(empty <= empty)
+  println(empty > empty)
+  println(empty >= empty)
+  println(a < a)
+  println(a <= a)
+  println(a > a)
+  println(a >= a)
+  println("")
+
+  val b = "b"
+
+  println(a < b)
+  println(a <= b)
+  println(a > b)
+  println(a >= b)
+  println(b < a)
+  println(b <= a)
+  println(b > a)
+  println(b >= a)
+  println("")
+
+  val ab = "ab"
+  println(ab < b)
+  println(ab <= b)
+  println(ab > b)
+  println(ab >= b)
+  println(b < ab)
+  println(b <= ab)
+  println(b > ab)
+  println(b >= ab)
+  println("")
+
+  println(a < ab)
+  println(a <= ab)
+  println(a > ab)
+  println(a >= ab)
+  println(ab < a)
+  println(ab <= a)
+  println(ab > a)
+  println(ab >= a)
+}

libraries/common/effekt.effekt

@@ -572,19 +572,19 @@ extern pure def infixGte(x: Double, y: Double): Bool =
 // TODO do we really need those? if yes, move to string.effekt
 extern pure def infixLt(x: String, y: String): Bool =
   js "(${x} < ${y})"
-  chez "(< ${x} ${y})"
+  chez "(string<? ${x} ${y})"
 
 extern pure def infixLte(x: String, y: String): Bool =
   js "(${x} <= ${y})"
-  chez "(<= ${x} ${y})"
+  chez "(string<=? ${x} ${y})"
 
 extern pure def infixGt(x: String, y: String): Bool =
   js "(${x} > ${y})"
-  chez "(> ${x} ${y})"
+  chez "(string>? ${x} ${y})"
 
 extern pure def infixGte(x: String, y: String): Bool =
   js "(${x} >= ${y})"
-  chez "(>= ${x} ${y})"
+  chez "(string>=? ${x} ${y})"
 
 
 // Boolean operations