Proposal: ArgumentDo

[David Luposchainsky]

Infix ($) is so noisy that I’ve adapted a code style that replaces it entirely
with parentheses. And the hack to make ($) work with higher-rank types is pretty
awkward too. Seen from these angles, I’m very much in favour of ArgumentDo.

However, the clarity of `runST do ...` comes at a high price: only some of the
missing ($) make the code clearer, where others make it much worse. You give a
couple of examples yourself:

> f
>   do x
>   do y

I have to think a bit how this might be parsed even after reading the proposal.
I’m a bit torn on whether I like or not like it, but I think the ArgumentDo
syntax that I find awkward now is a matter of style, and one could probably get
used to only using it in certain places, despite all of the possibilities.

David/quchen