add first-char, rest-char, slice

1 files changed, 35 insertions, 0 deletions

diff --git a/interpreter.py b/interpreter.py
index dbf8c82..04160cd 100644
--- a/interpreter.py
+++ b/interpreter.py
@@ -712,3 +712,38 @@ def interpretExists(symbol, args, env):
     return Literal(Path(file_or_dir.value).resolve().exists())
 
 GLOBALS.register("exists?", Builtin(interpretExists, 1))
+
+def interpretFirstChar(symbol, args, env):
+    ev = evaluate(args[0], env)
+    if not isinstance(ev, Literal):
+        raise Exception("'first-char' expects a string")
+    if len(ev.value) == 0:
+        raise Exception("string is empty")
+    return Literal(ev.value[0])
+
+GLOBALS.register("first-char", Builtin(interpretFirstChar, 1))
+
+def interpretRestChar(symbol, args, env):
+    ev = evaluate(args[0], env)
+    if not isinstance(ev, Literal):
+        raise Exception("'rest-char' expects a string")
+    return Literal(ev.value[1:])
+
+GLOBALS.register("rest-char", Builtin(interpretRestChar, 1))
+
+def interpretSlice(symbol, args, env):
+    lst = evaluate(args[0], env)
+    if not isinstance(lst, List):
+        raise Exception("'slice' expects a list as its first argument")
+    idx = evaluate(args[1], env)
+    if not isinstance(idx, Literal):
+        raise Exception("'slice' expects an integer as its second argument")
+    if len(args) == 2:
+        return List(lst.args[idx.value - 1:])
+    length = evaluate(args[2], env)
+    if not isinstance(length, Literal):
+        raise Exception("'slice' expects an integer as its third argument")
+    diff = idx.value - 1 + length.value
+    return List(lst.args[idx.value - 1:diff])
+
+GLOBALS.register("slice", Builtin(interpretSlice, 2, 3))