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> There are plenty of tricks you can not write on pure monads

This made me curious, could you list some?

Anything that needs rewriting a memory address without losing time by allocation and garbage collection.

Handling IO exceptions and masking them as different errors.

Thread, process, or machine coordination in parallel execution.

You don't need unsafePerformIO for any of these.

> Anything that needs rewriting a memory address without losing time by allocation and garbage collection.

ST gives you mutable references. Or just use IO.

You need it for doing what the OP is talking about.