From 9f89174ec62db63663eb9ab9f5cb2c22a6366967 Mon Sep 17 00:00:00 2001
From: Conor Bronsdon <120674402+conorbronsdon@users.noreply.github.com>
Date: Thu, 26 Mar 2026 23:05:53 -0700
Subject: [PATCH] fix(mojo-syntax): add string indexing and universal std.
 prefix to removed-syntax table

String indexing with byte= keyword and stdlib import prefix are the most
common compilation failures for Python developers new to Mojo. This adds
them prominently to the removed-syntax table and Strings section.

Findings from a structured skills evaluation building a real CLI project.
---
 mojo-syntax/SKILL.md | 28 ++++++++++++++++++++++++++++
 1 file changed, 28 insertions(+)

diff --git a/mojo-syntax/SKILL.md b/mojo-syntax/SKILL.md
index 90abb01..24a7db8 100644
--- a/mojo-syntax/SKILL.md
+++ b/mojo-syntax/SKILL.md
@@ -41,6 +41,11 @@ This skill specifically works on the latest Mojo, and stable versions may differ
 | `@register_passable("trivial")` | `TrivialRegisterPassable` trait |
 | `@register_passable` | `RegisterPassable` trait |
 | `Stringable` / `__str__` | `Writable` / `write_to` |
+| `s[i]` | `s[byte=i]` — returns `StringSlice`; wrap in `String()` if needed |
+| `s[0:10]`, `s[:5]` | No slice syntax on String — use `s.codepoint_slices()` or Python FFI |
+| `from sys import ...` | `from std.sys import ...` |
+| `from os import ...` | `from std.os import ...` |
+| `from pathlib import ...` | `from std.pathlib import ...` |
 | `from collections import ...` | `from std.collections import ...` |
 | `from memory import ...` | `from std.memory import ...` |
 | `constrained(cond, msg)` | `comptime assert cond, msg` |
@@ -184,6 +189,8 @@ var result = MyStruct(headers=d.copy())  # or: headers=d^
 
 ## Imports use `std.` prefix
 
+**All stdlib imports require the `std.` prefix.** The removed-syntax table shows the most common corrections, but the rule is universal.
+
 ```mojo
 from std.testing import assert_equal, TestSuite
 from std.algorithm import vectorize
@@ -365,6 +372,27 @@ v.reduce_min()                     # horizontal min → Scalar
 
 `len(s)` returns **byte length**, not codepoint count. Mojo strings are UTF-8. Byte indexing requires keyword syntax: `s[byte=idx]` (not `s[idx]`).
 
+### String indexing (common error)
+
+```mojo
+# WRONG — compile error
+var ch = s[0]
+var sub = s[0:10]
+
+# CORRECT — byte-level access
+var ch = s[byte=0]        # returns StringSlice
+var ch_str = String(s[byte=0])  # if you need a String
+
+# CORRECT — iterate codepoints for truncation
+var result = String("")
+var count = 0
+for cp in s.codepoint_slices():
+    if count >= 10:
+        break
+    result += String(cp)
+    count += 1
+```
+
 ```mojo
 var s = "Hello"
 len(s)                  # 5 (bytes)