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This is a relatively short chapter, here we demistify magic behind AST functor and associated natural transformations.
If you open Instruction page you'll find this operator that stands for covariant functor from Arrow into Arrow:
So for all objects, there exists such a functor t that makes Instruction a functor i.e. whether Instruction a functor or not depends on t.
From previous chapter we know for sure that Instruction is a functor - primarly because it was instantiated with a functor composition:
If you open this functor composition page you'll find something similar - underlying t and tt must be functors too:
Luckily for us, this is the case - all of them happen to be covariant functors:
And it works in exactly the same way for associated natural transformations too:
The latter is an example of mapping with target lax Kleisli. Next time we'll see how to trace variables with source lax Kleisli one.