allowed let this = x (#172)

lib/ast.js

@@ -996,19 +996,22 @@ exports.Call = Call = (function(superclass){
     return node.back = (args[index] = fun).body, node;
   };
   Call['let'] = function(args, body){
-    var params, res$, i, len$, a;
+    var params, res$, i, len$, a, that, gotThis;
     res$ = [];
     for (i = 0, len$ = args.length; i < len$; ++i) {
       a = args[i];
-      if (a.op === '=' && !a.logic) {
-        args[i] = a.right;
+      if (that = a.op === '=' && !a.logic && a.right) {
+        args[i] = that;
+        if (i === 0 && (gotThis = a.left.value === 'this')) {
+          continue;
+        }
         res$.push(a.left);
       } else {
         res$.push(Var(a.varName() || a.carp('invalid "let" argument')));
       }
     }
     params = res$;
-    args.unshift(Literal('this'));
+    gotThis || args.unshift(Literal('this'));
     return this.block(Fun(params, body), args, '.call');
   };
   return Call;

src/ast.co

@@ -608,10 +608,12 @@ class exports.Call extends Node
 
   @let = (args, body) ->
     params = for a, i of args
-      if a.op is \= and not a.logic
-      then args[i] = a.right; a.left
+      if a.op is \= and not a.logic and a.right
+        args[i] = that
+        continue if i is 0 and gotThis = a.left.value is \this
+        a.left
       else Var a.varName! || a.carp 'invalid "let" argument'
-    args.unshift Literal \this
+    gotThis or args.unshift Literal \this
     @block Fun(params, body), args, \.call
 
 #### List

test/function.co

@@ -445,6 +445,9 @@ eq '''
 ok let [it] = [ok]
   it is ok
 
+let this = eq
+  this eq, this
+
 
 ### `with`
 with o = {}