From c648ad597d34b2226f90efe8f8f951c5c17004e7 Mon Sep 17 00:00:00 2001
From: Jordan Orelli <jordanorelli@gmail.com>
Date: Tue, 30 Oct 2012 18:21:42 -0400
Subject: [PATCH] added cons, car, cdr

---
 proc.go  | 36 ++++++++++++++++++++++++++++++++++++
 skeam.go |  7 +++++++
 2 files changed, 43 insertions(+)

diff --git a/proc.go b/proc.go
index d6821d5..abef4cd 100644
--- a/proc.go
+++ b/proc.go
@@ -142,6 +142,42 @@ func issymbol(vals []interface{}) (interface{}, error) {
 	return ok, nil
 }
 
+func cons(vals []interface{}) (interface{}, error) {
+	if err := checkArity(2, vals, "cons"); err != nil {
+		return nil, err
+	}
+	s := sexp{vals[0]}
+	switch t := vals[1].(type) {
+	case sexp:
+		return append(s, t...), nil
+	default:
+		return append(s, t), nil
+	}
+	panic("not reached")
+}
+
+func car(vals []interface{}) (interface{}, error) {
+	if err := checkArity(1, vals, "car"); err != nil {
+		return nil, err
+	}
+	s, ok := vals[0].(sexp)
+	if !ok {
+		return nil, errors.New("expected list")
+	}
+	return s[0], nil
+}
+
+func cdr(vals []interface{}) (interface{}, error) {
+	if err := checkArity(1, vals, "cdr"); err != nil {
+		return nil, err
+	}
+	s, ok := vals[0].(sexp)
+	if !ok {
+		return nil, errors.New("expected list")
+	}
+	return s[1:], nil
+}
+
 type cmp_bin_i func(int64, int64) bool
 type cmp_bin_f func(float64, float64) bool
 
diff --git a/skeam.go b/skeam.go
index 6ee157d..0d9267f 100644
--- a/skeam.go
+++ b/skeam.go
@@ -35,12 +35,19 @@ var universe = &environment{map[symbol]interface{}{
 	">=":      builtin(gte),
 	"<":       builtin(lt),
 	"<=":      builtin(lte),
+	"cons":    builtin(cons),
+	"car":     builtin(car),
+	"cdr":     builtin(cdr),
 	"length":  builtin(length),
 	"list":    builtin(list),
 	"list?":   builtin(islist),
 	"not":     builtin(not),
 	"null?":   builtin(isnull),
 	"symbol?": builtin(issymbol),
+	// "="
+	// "equal?"
+	// "eq?"
+	// "append"
 
 	// special forms
 	"begin":  special(begin),