From d2d6829d4b0684275a662ac4ca5b0aaac8667b03 Mon Sep 17 00:00:00 2001
From: crumbtoo <crumb@disroot.org>
Date: Tue, 7 Nov 2023 15:00:21 -0700
Subject: [PATCH] core AST

---
 meson.build   |  5 +--
 rlp/Core.java | 88 +++++++++++++++++++++++++++++++++++++++++++++++++++
 rlp/RLPC.java | 22 +++++++++++++
 src/RLPC.java |  8 -----
 4 files changed, 113 insertions(+), 10 deletions(-)
 create mode 100644 rlp/Core.java
 create mode 100644 rlp/RLPC.java
 delete mode 100644 src/RLPC.java

diff --git a/meson.build b/meson.build
index a018a48..eb3d906 100644
--- a/meson.build
+++ b/meson.build
@@ -7,7 +7,8 @@ project(
 
 rlpc_jar = jar(
   'rlpc',
-  'src/RLPC.java',
-  main_class: 'RLPC'
+  'rlp/RLPC.java',
+  'rlp/Core.java',
+  main_class: 'rlp.rlpc.RLPC'
 )
 
diff --git a/rlp/Core.java b/rlp/Core.java
new file mode 100644
index 0000000..158176a
--- /dev/null
+++ b/rlp/Core.java
@@ -0,0 +1,88 @@
+package rlp;
+/*------------------------------------------------------------------------------*/
+import java.util.function.BiFunction;
+/*------------------------------------------------------------------------------*/
+
+public class Core
+{
+    /* day 1: boy do i miss Haskell */
+    public sealed interface Expr permits App, TyApp, Abs, TyAbs, Var {}
+    public record App(Expr f, Expr x)       implements Expr {}
+    public record TyApp(Expr f, Ty x)       implements Expr {}
+    public record Abs(Name n, Ty t, Expr m) implements Expr {}
+    public record TyAbs(Name n, Expr m)     implements Expr {}
+    public record Var(Name n)               implements Expr {}
+
+    sealed interface Ty permits TyVar, Forall, TyFunc {}
+    public record TyVar(Name n)         implements Ty {}
+    public record Forall(Name n, Ty t)  implements Ty {}
+    public record TyFunc(Ty a, Ty b)    implements Ty {}
+
+    public final static class Name
+    {
+        public final String s;
+        public Name(String s)
+        {
+            // TODO: verify that `s` is a valid identifier
+            this.s = s;
+        }
+    }
+
+    public static String showExpr(Expr e)
+    {
+        return showExprP(0, e);
+    }
+
+    static String showExprP(int p, Expr e)
+    {
+        return switch(e)
+        {
+            /* prec: 1 */
+            case App a ->
+                wrap(p, 1, showExprP(2, a.f) + " " + showExprP(2, a.x));
+
+            /* prec: 1 */
+            case TyApp a ->
+                wrap(p, 1, showExprP(2, a.f) + " " + showTyP(2, a.x));
+
+            /* prec: 0 */
+            case Abs a ->
+                wrap(p, 0, String.format
+                    ( "λ(%s:%s).%s"
+                    , a.n.s
+                    , showTyP(0, a.t)
+                    , showExprP(0, a.m)));
+
+            /* prec: 0 */
+            case TyAbs a ->
+                wrap(p, 0, String.format
+                    ( "Λ%s.%s"
+                    , a.n.s
+                    , showExprP(0, a.m)));
+
+            case Var a ->
+                a.n.s;
+        };
+    }
+
+    static String showTy(Ty t)
+    {
+        return showTyP(0, t);
+    }
+
+    static String showTyP(int p, Ty t)
+    {
+        return switch(t)
+        {
+            case TyVar a   -> a.n.s;
+            case Forall a  -> "";
+            case TyFunc a  -> "";
+        };
+    }
+
+    static String wrap(int p, int n, String s)
+    {
+        return (p <= n) ? s : "(" + s + ")";
+    }
+}
+
diff --git a/rlp/RLPC.java b/rlp/RLPC.java
new file mode 100644
index 0000000..a2db769
--- /dev/null
+++ b/rlp/RLPC.java
@@ -0,0 +1,22 @@
+package rlp.rlpc;
+/*----------------------------------------------------------------------------*/
+import rlp.Core;
+/*----------------------------------------------------------------------------*/
+
+class RLPC
+{
+	public static void main(String[] argv)
+	{
+		// final Core.Expr e = new Core.App(new Core.Var(new Core.Name("f")), new Core.Var(new Core.Name("x")));
+		final Core.Expr e =
+			new Core.App
+				( new Core.Abs
+					( new Core.Name("x")
+					, new Core.TyVar(new Core.Name("α"))
+					, new Core.Var(new Core.Name("x"))
+					)
+				, new Core.Var(new Core.Name("y")));
+		System.out.println(Core.showExpr(e));
+	}
+}
+
diff --git a/src/RLPC.java b/src/RLPC.java
deleted file mode 100644
index e593dd5..0000000
--- a/src/RLPC.java
+++ /dev/null
@@ -1,8 +0,0 @@
-class RLPC
-{
-	public static void main(String[] argv)
-	{
-		System.out.println("hello worms");
-	}
-}
-