Is join useful for every monad?

[Carter Schonwald]

Join actually also comes up in compiler engineering!

Most normalized compiler reps: notably anf and cps, have a sort of flatness
condition where you can’t have nested subexpressions (aka in many cases in
strict languages this is where evaluation order becomes explicit ) and the
join operation corresponds to a step in the flattening process for nested
expression syntax when you do compiler transformations in this setting.

This is in fact exactly why it’s pretty brutal to write the monad for an
anf or cps syntax , you’re essentially specifying subexpression evaluation
order for all pairs of syntax constructors!

 And while join is not at the moment in the Monad typeclass because of
newtype stuff, writing these monad instances is way saner in terms of the
join operators rather than in terms of bind. At least in my biased
perspective ;)

On Mon, Oct 14, 2019 at 4:37 AM Georgi Lyubenov <godzbanebane at gmail.com>
wrote:

> In general, if you want to *dynamically generate* actions depending on
> the result of an earlier action you will always encounter join/(>>=).
> For example (with ReadPrec/Parser):
> I want to first parse a character, and then parse the same character two
> more times.
> numberAndThenThatManyAs = join (fmap (\c -> satisfy (==c) *> satisfy
> (==c)) char)
>
> Of note:
> * The example is contrived for simplicity's sake, but you do really need a
> Monad (and hence join) to perform stuff like this in general. A more
> practical example would be parsing command-line options that depend on
> previous options.
> * Obviously it's way more humane to write this with do-syntax. (or (>>=)
> or something) - do { c <- char; satisfy (==c); satisfy (==c) }
> * I'm not actually sure whether you need a Monad in this situation, maybe
> you could get away with just selectives
> <http://hackage.haskell.org/package/selective-0.3>
>
> =======
>
> Georgi
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