From 807f9818a5579e318edf7b50d70470d8b8a42382 Mon Sep 17 00:00:00 2001
From: Calvin Rose <calsrose@gmail.com>
Date: Sun, 11 Mar 2018 16:30:38 -0400
Subject: [PATCH] Add let macro.

---
 src/assembler/asm.c   |  2 +-
 src/compiler/boot.dst | 16 +++++++++++++++-
 test/suite0.dst       |  6 ++++++
 3 files changed, 22 insertions(+), 2 deletions(-)

diff --git a/src/assembler/asm.c b/src/assembler/asm.c
index 709708da..44bf06d4 100644
--- a/src/assembler/asm.c
+++ b/src/assembler/asm.c
@@ -704,7 +704,7 @@ DstAssembleResult dst_asm(Dst source, int flags) {
  * NULL if not found. */
 static const DstInstructionDef *dst_asm_reverse_lookup(uint32_t instr) {
     size_t i;
-    uint32_t opcode = instr & 0xFF;
+    uint32_t opcode = instr & 0x7F;
     for (i = 0; i < sizeof(dst_ops)/sizeof(DstInstructionDef); i++) {
         const DstInstructionDef *def = dst_ops + i;
         if (def->opcode == opcode) 
diff --git a/src/compiler/boot.dst b/src/compiler/boot.dst
index 36fe7cec..db0cbdb5 100644
--- a/src/compiler/boot.dst
+++ b/src/compiler/boot.dst
@@ -90,7 +90,21 @@
     (tuple 'def endsym end)
     (tuple 'while (tuple '< sym endsym)
      (tuple-prepend body 'do)
-     (tuple 'varset! sym (tuple '+ sym 1)))))
+     (tuple 'varset! sym (tuple '+ sym inc)))))
+
+(defn even? [x] (== 0 (% x 2)))
+(defn odd? [x] (== 1 (% x 2)))
+
+(defmacro let [bindings & body]
+  (def head (ast-unwrap1 bindings))
+  (when (odd? (length head)) (error "expected even number of bindings to let"))
+  (var accum ['do])
+  (for [i 0 (length head) 2]
+   (array-push accum (tuple 'def 
+                     (get head i)
+                     (get head (+ 1 i)))))
+  (array-push accum (tuple-prepend body 'do))
+  (apply tuple accum))
 
 (defn pairs [x]
  (var lastkey (next x nil))
diff --git a/test/suite0.dst b/test/suite0.dst
index 506b152c..4dc7730b 100644
--- a/test/suite0.dst
+++ b/test/suite0.dst
@@ -259,6 +259,12 @@
 		(varset! count (+ 1 count)))
 	(assert (= (length syms) 128) "many symbols")))
 
+# Let
+
+(assert (= (let [a 1 b 2] (+ a b)) 3), "simple let")
+(assert (= (let [[a b] [1 2]] (+ a b)) 3), "destructured let")
+(assert (= (let [[a [c d] b] [1 (tuple 4 3) 2]] (+ a b c d)) 10), "double destructured let")
+
 # Macros
 
 (def defmacro macro (fn [name & more]