From b316404b2bba864524dc202de8353391438217a5 Mon Sep 17 00:00:00 2001
From: MadMaurice <madmaurice@zom.bi>
Date: Sat, 3 Apr 2021 18:05:59 +0200
Subject: [PATCH] Add cons, car, cdr

---
 lib/Minilisp.pm | 15 ++++++++++++++-
 t/lists.t       | 12 ++++++++++++
 2 files changed, 26 insertions(+), 1 deletion(-)

diff --git a/lib/Minilisp.pm b/lib/Minilisp.pm
index 52c5da8..9c29af1 100644
--- a/lib/Minilisp.pm
+++ b/lib/Minilisp.pm
@@ -285,7 +285,20 @@ my %stdctx = (
     $v = $cb->($v,$_) foreach (@copy);
     return $v;
   },
-  # TODO: this is tricky as it needs to recurse into sub lists
+  'cons' => sub {
+    my ($v, $list) = @_;
+    return [ $v, @$list ];
+  },
+  'car' => sub {
+    my ($list) = @_;
+    return $list->[0];
+  },
+  'cdr' => sub {
+    my ($list) = @_;
+    my @newlist = @$list;
+    shift @newlist; # drop first element
+    return \@newlist;
+  },
 
   # Constants
   't' => 1,
diff --git a/t/lists.t b/t/lists.t
index 6835252..806115d 100644
--- a/t/lists.t
+++ b/t/lists.t
@@ -16,3 +16,15 @@
 
 (expect "equal tests recursively 2"
         (not (equal (list (list 1) (list 2)) (list (list 1) (list 3)))))
+
+(expect "Length of empty list is 0"
+        (zerop (length (list))))
+
+(expect "cons adds elements to the front"
+        (equal (list 1 2) (cons 1 (list 2))))
+
+(let ((lst (list 5 1 2)))
+  (expect "car gives first element"
+          (= 5 (car lst)))
+  (expect "cdr gives rest of list"
+          (equal (list 1 2) (cdr lst))))