Implement let*

lib/Minilisp.pm

@@ -512,13 +512,15 @@ sub parser_call {
   };
 }
 
-sub macro_let {
+sub gen_macro_let {
+  my $incremental = shift;
+  return sub {
     my $ts = shift;
 
     my $tok = shift @$ts;
     die "Expected ( after let" unless $tok->{type} == LPAREN;
 
-  my $variables = {};
+    my @assignments;
 
     while ($ts->[0]->{type} != RPAREN)
       {
@@ -527,20 +529,23 @@ sub macro_let {
 
         my $ident = parser_identifier($ts);
 
-      my $assignment;
+        my $expr;
         if ($ts->[0]->{type} == RPAREN)
           {
-          $assignment = sub { return undef };
+            $expr = sub { return undef };
           }
         else
           {
-          $assignment = parser_expr($ts)
+            $expr = parser_expr($ts)
           }
 
         $tok = shift @$ts;
         die "Expected ) after parameter pair" unless $tok->{type} == RPAREN;
 
-      $variables->{$ident} = $assignment;
+        push @assignments, {
+          ident => $ident,
+          expr => $expr
+        };
       }
 
     die "Expected ) after parameter list in let" unless $ts->[0]->{type} == RPAREN;
@@ -551,12 +556,14 @@ sub macro_let {
     return sub {
       my $octx = shift;
       my $ictx = ctx_create($octx);
-    ctx_set($ictx, $_, $variables->{$_}->($octx)) foreach (keys %$variables);
+      ctx_set($ictx, $_->{ident}, $_->{expr}->($incremental ? $ictx : $octx)) foreach (@assignments);
 
       return $inner->($ictx);
-  }
+    };
+  };
 }
-$macros{let} = \&macro_let;
+$macros{let} = gen_macro_let(0);
+$macros{'let*'} = gen_macro_let(1);
 
 sub macro_lambda {
   my $ts = shift;

t/let.t

@@ -2,3 +2,17 @@
   (expect "a is 5" (= a 5))
   (expect "b is 6" (= b 6))
   )
+
+(let ((a 'top))
+  (let ((a 'shadow)
+        (b a))
+    (comment b)
+    (expect "let - non incremental context"
+            (equal b 'top))))
+
+(let ((a 'top))
+  (let* ((a 'shadow)
+         (b a))
+    (comment b)
+    (expect "let* - Incremental context"
+            (equal b 'shadow))))