From b395d5d1cd122bb07017f23110f76249398a61bb Mon Sep 17 00:00:00 2001
From: mryouse
Date: Sat, 4 Jun 2022 02:05:32 +0000
Subject: add first-char, rest-char, slice

---
 interpreter.py | 35 +++++++++++++++++++++++++++++++++++
 1 file changed, 35 insertions(+)

(limited to 'interpreter.py')

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))
-- 
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