Change the module combinator `flip` to an interface combinator

[kjh618]

(From the discussions on the Oct. 14 class) Would it be better to change the module combinator flip to an interface combinator that just flips a tuple interface? Then with a function composition operator ∘ (is this comb?), we can do flip ∘ m ∘ flip to flip both the ingress and the egress interface.

[minseongg]

Actually, we can represent the interface version of flip which just flips a tuple interface as follows:

(i, j).comb(|(i1, i2)| (i2, i1))

Thus, given a module m: impl FnOnce(I1, I2) -> (O1, O2), we can represent flip(m): impl FnOnce(I2, I1) -> (O2, O1) using interface combinators like this:

(i2, i1)
    .comb(|(i2, i1)| (i1, i2))  // flip
    .comb(|(i1, i2)| m(i1, i2)) // m
    .comb(|(o1, o2)| (o2, o1))  // flip

It is conceptually the same as flip ∘ m ∘ flip, as you mentioned.

So, you can just think of flip as a helper module combinator which wraps the above sequence of comb s.