[bug#61214] [PATCH guix-artwork v3] website: posts: Add Dissecting Guix, Part 2: The Store Monad.

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Hi,

On Sun Feb 12, 2023 at 10:47 AM GMT, Christopher Baines wrote:
> I think there's some room to improve the introduction here. Linking to
> the previous post in the series is fine, but what I think is missing is
> some context around the topic and setting some expectations for the
> reader.

> I'm not sure who you're pitching this post at

People who have used Guix and know basic Scheme but haven't delved into Guix's
interiors yet :)

> I think the s's after the `#f` and `'()` here don't aid
> readability. Something like:

Fair.

> I think it would be clearer to say "To define the maybe monad, we use
> the define-monad macro.", then there's no need to keep track of what API
> is being discussed. I'm also not sure it's useful to talk about things
> within Guix as APIs unless you're talking about a specific case of using
> Guix from some external program/software.

Good point.  Maybe I could say something like:

  "To define the maybe monad's behaviour, we use the define-monad macro."

using "behaviour" to describe the specifics of a monad.

> I think this would be confusing for someone who's encountering monads
> for the first time. I think it's good to try and avoid going to deep,
> but if there's mention of the "laws", I think it's important to say that
> these laws come from category theory.

Yeah, okay.

> > +
> > +```scheme
> > +(mbegin %maybe-monad
> > +  (remove-a "abc"))
> > +;; #<<maybe> is?: #t value: "bc">
> > +```
>
> This is stretching my understanding of monads here, but would this
> example be better if the (mbegin bit included two expressions rather
> than one?

I might just remove the MBEGIN example entirely.  I have no idea why MBEGIN exists,
or what advantages it confers, so I just included it for the sake of completeness
-.o.-  If someone could elaborate on what MBEGIN is for I would very much appreciate
it.

> I think the point is still good here, but maybe it's simpler to say "but
> why does Guix use monads?".

Okay.

> > +So, when we do `(run-with-state result (list 32))`, we're passing `(list 32)` as
> > +the initial state value, and then the `>>=` form passes that and `33` to
> > +`state-push`.  What `%state-monad` allows us to do is thread together some
> > +procedures that require some kind of state, while pretending the state isn't
> > +there, and then retrieve both the final state and the result at the end!
>
> I'm not sure the "pretending the state isn't there" but is helpful here,
> if you're pretending the state doesn't exist, why is writing monadic
> code helpful?

Yeah, this doesn't really get across the point I'm trying to make.  I'm not sure
how else to word it, though...

> > +We mentioned that, technically, we didn't need monads for Guix.  Indeed, many
> > +(now deprecated) procedures take a store value as the argument, such as
> > +`build-expression->derivation`.  However, using monads both helps ensure purity
> > +and simply looks nicer.
>
> I'm not sure what you mean by purity here?

Me neither :P  Simon mentioned something about monads ensuring purity in their
review, which I didn't quite understand, so I just wrote something vague about it
(which I shouldn't have done).

> > +And indeed, it symlinks the `irssi` binary to the output path.  Some other,
> > +higher-level, monadic procedures include `interned-file`, which copies a file
> > +from outside the store into it, and `text-file`, which copies some text into it.
> > +Generally, these procedures aren't used, as there are higher-level procedures
> > +that perform similar functions (which we will discuss later), but for the sake
> > +of this blog post, here's an example:
> > +
> > +```scheme
> > +(with-store store
> > +  (run-with-store store
> > +    (text-file "unmatched-paren"
> > +      "( <paren@disroot.org>")))
> > +;; "/gnu/store/v6smacxvdk4yvaa3s3wmd54lixn1dp3y-unmatched-paren"
> > +```
>
> I think the build up to this section is pretty good, but then I'm not
> sure what this last section is trying to explain.

It's just showing an example of the TEXT-FILE procedure, that's all :)

> Maybe at this point it would be good to leave the REPL and give some
> concrete examples of non-trivial monadic code in Guix, and discuss what
> that would look like if implemented without using monads.

Good idea! :)

> > +# Conclusion
> > +
> > +What have we learned about monads?  The key points we can take away are:
> > +
> > +1. Monads are a way of composing together procedures and values that are wrapped
> > +   in containers that give them extra context, like `maybe` values.
> > +2. Guix provides a high-level monad API that compensates for Guile's lack of
> > +   strong types or an interface-like system.
>
> I'd say that Guile is a strongly typed language. I'm also not sure what
> the point about compensating for something lacking in Guile means.

Guile doesn't have type definitions and it can't "fix" values to types.  I'd
consider that to be weak typing, personally :)

Regarding the point: it's supposed to say something like

> > +4. Guix uses the store monad frequently to thread a store connection through
> > +   procedures that need it.
> > +5. The store monad is really just the state monad in disguise, where the state
> > +   value is used to thread the store object through monadic procedures.
>
> 4 and 5 here are observations, but not very useful conclusions. I think
> the more interesting question to ask is why are things implemented this
> way?

> Ideally the closing points would be well made in the previous section,
> and this final bit would be a summary.

They're supposed to be a short summary of the main lessons the blog post
attempts to teach, but I'll consider removing them.

> > +If you've read this post in its entirety but still don't yet quite get it, don't
> > +worry.  Try to modify and tinker about with the examples, and hopefully it will
> > +all click eventually!
>
> Maybe this could be a call to get involved in the community (talk on IRC
> or the mailing list?

Yeah, good idea :)

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