Allow easier aliasing.

Rather than using "(set! new-name old-name)", which works, we should
use an "(alias new-name old-name)" function which expresses our intent
more cleanly.

This closes #38.

eval/eval.go

@@ -500,6 +500,24 @@ func (ev *Eval) eval(exp primitive.Primitive, e *env.Environment, expandMacro bo
 			// first token/symbol
 			switch listExp[0] {
 
+			// (alias ..)
+			case primitive.Symbol("alias"):
+				if len(listExp) != 3 {
+					return primitive.Error("Expected two arguments")
+				}
+
+				// Name we'll use
+				name := listExp[1]
+
+				// Existing function.
+				val := listExp[2]
+
+				old, ok := e.Get(val.ToString())
+				if ok {
+					e.Set(name.ToString(), old)
+				}
+				return primitive.Nil{}
+
 			// (do ..)
 			case primitive.Symbol("do"):
 				var ret primitive.Primitive

eval/eval_test.go

@@ -194,6 +194,10 @@ a
 		{`(split "steve" "")`, `(s t e v e)`},
 		{`(join (list "s" "t" "e" "v" "e"))`, `steve`},
 
+		// alias
+		{`(alias explode split) (explode "steve" "")`, `(s t e v e)`},
+		{`(alias ** #) (** 3 3)`, `27`},
+
 		// comparison
 		{"(< 1 3)", "#t"},
 		{"(< 10 3)", "#f"},
@@ -203,7 +207,7 @@ a
 		{"(= 10 10)", "#t"},
 		{"(= -1 -1)", "#t"},
 
-		// we have a LOT of built ins, but not 100
+		// we have a LOT of built ins, but not 200
 		{"(> (length (env))  10)", "#t"},
 		{"(> (length (env))  50)", "#t"},
 		{"(< (length (env)) 200)", "#t"},
@@ -266,6 +270,8 @@ a
 		{"{ :age 333  ", "nil"},
 		{"}}}}}}", "nil"},
 
+		{"(alias foo)", "ERROR{Expected two arguments}"},
+
 		// try / catch
 		{"(try 3)", "ERROR{arity-error: not enough arguments for (try ..)}"},
 		{"(try 3 3)", "ERROR{expected a list for argument, got 3}"},