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There is now one such variable (function variable, which must be of type a = > a):

const fn = a => a

Now the fn function reads an external variable:

import op from 'op'

// op: a => a
const fn = a => op(a)

So the fn function is not pure, change it like this:

const fn = (op, a) => op(a)

But fn must be of type a = > a, then a helper function is required:

const pure = fn => op => a => fn(op, a)

const func = pure((op, a) => op(a))

const fn = func(op)

This pure function takes (op, a) = > an output op = > a = > a type. If you now need to modify the operation within func, such as op (a) becomes op (a + a), you need another helper function:

const fmap = join => func => pure(join(func))

const func2 = fmap(func => (op, a) => func(op)(a + a))

const fn = func2(func)(op)

Summary: If a function needs to have side effects, write a function that generates Monad, and then write a function that operates Monad and a function that executes Monad.