coding-mojo

Mojo Development in Claude.ai Containers

Mojo is a systems programming language from Modular that combines Python-like syntax with C-level performance. This skill handles container setup and execution. For language syntax and semantics, defer to Modular's official skills at github.com/modular/skills — they are authoritative correction layers for pretrained knowledge.

Installation

Install once per session (~20s via uv, ~500MB). Skip if already installed.

if mojo --version 2>/dev/null; then
  echo "Mojo already installed"
else
  # Compiler binary without ML extras (~350MB saved)
  uv pip install --system --break-system-packages modular --no-deps 2>&1 | tail -5
  # Entry points + base deps (numpy, pyyaml, rich)
  uv pip install --system --break-system-packages mojo max 2>&1 | tail -5
  mojo --version
fi

Verify:

echo 'def main(): print("Mojo ready")' > /tmp/_verify.mojo && mojo /tmp/_verify.mojo

Running Mojo Code

Quick tests (write to temp file):

cat > /tmp/test.mojo << 'EOF'
def main():
    print("hello")
EOF
mojo /tmp/test.mojo

File execution (JIT compile + run, ~1.4s overhead):

cat > /home/claude/example.mojo << 'EOF'
def main():
    print("Hello from Mojo")
EOF
mojo /home/claude/example.mojo

Build binary (for benchmarking — ~6s cold compile, but binary runs at native speed):

mojo build /home/claude/example.mojo -o /home/claude/example
/home/claude/example

Use mojo build for benchmarks — mojo (JIT) includes ~1.4s compilation overhead per run. There is no mojo -e flag; always write to a file.

Critical Syntax Corrections (v26.2)

Pretrained models generate outdated Mojo. These corrections are current as of Mojo 26.2:

Wrong (pretrained)	Correct (26.2)	Notes
fn main():	def main():	fn is deprecated; def is the only function keyword
let x = 5	var x = 5	let removed; var for all bindings
inout self	mut self / out self	mut for mutation, out for __init__
@parameter for	comptime for	Compile-time loops
List[Int](1, 2, 3)	[1, 2, 3]	Collection literals
from math import sqrt	from std.math import sqrt	std. prefix required for all stdlib modules
from time import X	from std.time import X	Includes perf_counter_ns, sleep, etc.
__str__ / Stringable	write_to / Writable	String conversion protocol
String(self.x) for int→str	String(self.x)	This one is actually correct, but str() is not
list.append(item)	list.append(item^)	Non-copyable types require ^ transfer operator
var x: Int = perf_counter_ns()	var x: UInt = perf_counter_ns()	Time functions return UInt, not Int
Implicit copy of List[T]	.copy() or ^ transfer	List is not implicitly copyable; use explicit copy or move