`begin` is now a special form.

Two reasons:

1. Making it a builtin defeats tail call optimization, as builtins do not participate in TCO.

2. The `begin` form is occasionally generated by the macro expander, and should not rely on library support.

2 files changed, 5 insertions, 6 deletions

diff --git a/actinide/core.py b/actinide/core.py
index 1b4c800..467f04f 100644
--- a/actinide/core.py
+++ b/actinide/core.py
@@ -3,12 +3,6 @@ from .builtin import make_registry
 
 ACTINIDE_BINDINGS, ACTINIDE_VOIDS, ACTINIDE_FNS, ACTINIDE_BUILTINS, bind, void, fn, builtin = make_registry()
 
-@fn
-def begin(*args):
-    if args:
-        return args[-1]
-    return None
-
 @builtin
 def values(*args):
     return args
diff --git a/actinide/evaluator.py b/actinide/evaluator.py
index af25908..fe2e4c2 100644
--- a/actinide/evaluator.py
+++ b/actinide/evaluator.py
@@ -101,6 +101,9 @@ def branch(on_true, on_false):
 def append(args, continuation):
     return lambda environment, *tail: (continuation, environment, *args, *tail)
 
+def begin(continuation):
+    return lambda environment, *args: (continuation, environment, *(args[-1:] if args else ()))
+
 # Transforms a continuation which should receive function results into a
 # function call continuation. A function call continuation receives a function
 # and a sequence of arguments. If the function is a primitive function, the
@@ -171,6 +174,8 @@ def eval(value, symbols, continuation):
         return define(t.tail(value), symbols, continuation)
     if t.head(value) == symbols['lambda']:
         return lambda_(t.tail(value), symbols, continuation)
+    if t.head(value) == symbols['begin']:
+        return apply(t.tail(value), symbols, begin(continuation))
     # Ran out of alternatives, must be a function application
     return apply(value, symbols, invoke(continuation))