IO Operators

src/repl/Repl.kt

@@ -1,4 +1,90 @@
 package repl
 
+import io.Logger
+import io.Terminal
+import parser.ReplParser
+import prolog.Answer
+import prolog.Answers
+
 class Repl {
+    private val io = Terminal()
+    private val parser = ReplParser()
+
+    fun start() {
+        io.say("Prolog REPL. Type '^D' to quit.\n")
+        while (true) {
+            try {
+                printAnswers(query())
+            } catch (e: Exception) {
+                Logger.error("Error parsing REPL: ${e.message}")
+            }
+        }
+    }
+
+    fun query(): Answers {
+        val queryString = io.prompt("?-", { "" })
+        val query = parser.parse(queryString)
+        return query.satisfy(emptyMap())
+    }
+
+    fun printAnswers(answers: Answers) {
+        val knownCommands = setOf(";", "a", ".", "h")
+
+        if (answers.none()) {
+            io.say("false.")
+        } else {
+            val iterator = answers.iterator()
+            var previous = iterator.next()
+            io.say(prettyPrint(previous))
+
+            while (iterator.hasNext()) {
+                var command = io.prompt("")
+
+                while (command !in knownCommands) {
+                    io.say("Unknown action: $command (h for help)\n")
+                    command = io.prompt("Action?")
+                }
+
+                when (command) {
+                    ";" -> previous = iterator.next()
+                    "a" -> return
+                    "." -> return
+                    "h" -> {
+                        help()
+                        io.say("Action?")
+                    }
+                }
+            }
+
+            io.say(prettyPrint(previous))
+        }
+
+        io.say("\n")
+    }
+
+    fun help(): String {
+        io.say("Commands:\n")
+        io.say("  ;   find next solution\n")
+        io.say("  a   abort\n")
+        io.say("  .   end query\n")
+        io.say("  h   help\n")
+        return ""
+    }
+
+    fun prettyPrint(result: Answer): String {
+        result.fold(
+            onSuccess = {
+                val subs = result.getOrNull()!!
+                if (subs.isEmpty()) {
+                    return "true."
+                }
+                return subs.entries.joinToString(",\n") { "${it.key} = ${it.value}" }
+            },
+            onFailure = {
+                val text = "Failure: ${it.message}"
+                Logger.warn(text)
+                return text
+            }
+        )
+    }
 }