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Mojo gives you both def and fn. Picking well means matching the tool to what your code actually needs. 🧭

Pick def when you are exploring, prototyping, or want Python-like ease without committing to types just yet.

Pick fn when speed and safety matter: tight loops, library code, and anywhere the compiler should verify your types.

A common path is to sketch logic with def, then rewrite the hot parts as fn once the design settles.

With def you trade compile-time checks for freedom. With fn you trade freedom for early error detection.

An fn signature reads like documentation: its types tell the next person exactly what goes in and what comes out.

You can call an fn from a def and the other way around. They live happily together in the same Mojo file.

When juggling dynamic Python objects through interop, a def can be smoother, since types may not be known upfront.

Numeric kernels and inner loops belong in fn, where known types let Mojo generate the tightest possible code.

You can always begin with whichever feels natural. Switching a def to an fn later is a small, safe edit.

If you would benefit from the compiler checking types and squeezing out speed, write fn; otherwise def keeps things light.

Match the situation to the better function style.

Reach for def when you want flexibility and quick iteration, and fn when you need type safety and speed. 🎯