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fix(mojo-syntax): add string indexing and universal std. prefix corrections #5

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fix(mojo-syntax): add string indexing and universal std. prefix corrections #5

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28 changes: 28 additions & 0 deletions mojo-syntax/SKILL.md
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Original file line number Diff line number Diff line change
Expand Up @@ -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` |
Expand Down Expand Up @@ -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
Expand Down Expand Up @@ -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)
Expand Down