Boilerplate busting in functional languages

https://lambdaland.org/posts/2024-05-01_definitely_not_about_monads/

by ashton314 • 2 years ago

27 28 2 years ago

chowells• 2 years ago

This severely underestimates the weirdness of the visitor pattern. It's far more complicated than monads.

Smaug1233• 2 years ago

I claim the visitor pattern isn't that weird. The `visit` function is the natural way to move from continuation-passing style to direct style, being of type signature `forall 'ret . (MyObject -> 'ret) -> 'ret`.

mmaniac• 2 years ago

Explanations of monads are vastly more complicated than monads. Any Javascript programmer will recognise what .flatMap() is and does - you don't even need to know design patterns for that.

Smaug1233• 2 years ago

So the wise soul does without doing, teaches without talking.

Ursula Le Guin translating the Tao Te Ching, verse 2.

munchler• 2 years ago

In languages that don’t have typeclasses, you need to take special steps to ensure that you dispatch to the proper variant of the monad.

You might want to take a look at F#, which has no typeclasses (yet) but does have excellent first-class support for monadic syntax via "computation expressions".

https://learn.microsoft.com/en-us/dotnet/fsharp/language-ref...

williamcotton3• 2 years ago

Here's something I whipped up to show an example of a ResultWriter monad using computation expressions in F#. I'm sure there are some slightly better interfaces than this, and I wouldn't mind a code review if anyone is feeling helpful!

https://gist.github.com/williamcotton/95ebc21965e393f4c99a59...

fire_lake• 2 years ago

You can probably use a more efficient data structure than a singly linked list for logs

Maybe a “string list list”?

Smaug1233• 2 years ago

By the way, a classic:

A functional programmer is one who calls their variables `f` and `x`, and their design patterns "zygohistomorphic prepromorphism".

throwup238• 2 years ago

May their proverbs be as inscrutable as their code, amen.

hi-v-rocknroll3• 2 years ago

Sounds like some Dunning-Kruger inability to grasp how to solve the problem working with the tools rather than imposing their ideas that don't fit. If you need a "global mutable variable" in BEAM land, then you could just use a genserver or ETS. It depends if the solution needs to be a cluster-aware singleton or not.

In the case of business logic and Elixir, this is where you may need to write your own DSL to model domain logic (DDD). It makes it makes things more flexible and more maintainable than piling together mud and hoping it won't fall over and that someone else in the future will be able to reason about it and maintain it.

ashton3144• 2 years ago

I say "global mutable variable" to introduce the problem, followed by "global mutable state" in the summary of the first solution. The evil isn't a variable, it's just global mutable state in general. In cases where you need state, ETS and GenServers are good solutions—we agree there. But are you suggesting that the right way to solve this problem would be to spin up a GenServer every time you need to run what should be a pure function? I'd almost rather use the process dictionary!

What I'm saying is, this problem shouldn't need state to solve it, therefore any introduction of state is a hack that will be a headache to maintain, test, and reason about.

In the case of business logic and Elixir, this is where you may need to write your own DSL to model domain logic (DDD). It makes it makes things more flexible and more maintainable than piling together mud and hoping it won't fall over and that someone else in the future will be able to reason about it and maintain it.

I don't know what you're referring to here. What problem would a DSL solve (and how?), and what do you consider to be the mud that's being piled on?

hi-v-rocknroll3• 2 years ago

How else are you going to get global atomic, possibly highly-available share nothing mutable state? Magic? GenServers aren't difficult. Use the patterns rather than wincing or complaining about the lack of nonexistent wishes if you're not going to build a simpler solution.

Business rules are often represented as an ordering of constraints and triggers in models when using DDD & MVC. One example in another language would be trailblazer.[0]

Mud is when gobs of fragile, low-level of code is slapped together just to solve today's problem where everything is a special case without testability, architecture, standardization, or concern for maintenance or extensibility.

1. https://en.wikipedia.org/wiki/Business_rules_engine

recursive6• 2 years ago

This could work, but the fact that we are using global mutable state ...

What? Why would you consider using global state for this? I'd do it like this. Mutability without globals.

    export function transform(ast) {
        const queries = [];
        walk(ast.root, queries);
        return { ast, queries };
    }

    function walk(node, queries) {
        // recursively called on child nodes
        // can mutate node, and append to queries
    }

> Surprise! This is exactly the writer monad! This whole post has been a monad tutorial in disguise!

Aha. That explains why it stopped making sense about 6 paragraphs ago.

mrkeen4• 2 years ago

The label 'global mutable state' has a slightly broader reach in FP. Just because you don't see a 'static int' hanging out in the space between methods doesn't mean that you haven't given the whole world mutable access into your state.

walk can mutate node. transform can invoke walk. transform is public.

recursive6• 2 years ago

Whenever we call our transform function, we would have to ensure that we clear out the old list of accumulated information.

This is why I presumed this post is using the popular understanding of the term.

Anyway, nodes are only public if the calling code makes them public by leaking a reference or something.

Don’t forget about all the other problems that global mutable state brings. There must be a better way.

When this blog says "global mutable state", then it must be referring to what's commonly understood to be "mutable state". It seems to be an article of faith that there must be a better way. To my sensibility, that is the best way. The argument seems to rest on some sleight-of-hand regarding "global".

ashton3144• 2 years ago

You're right that you could create some local state and mutate that. That would be a better solution than some dumb global variable sitting somewhere. I considered touching on that, but this was already a long blog post!

I'd still be a little worried about your above solution though—I'd have to be sure of the state semantics in $LANGUAGE and I'd want to make sure the compiler wouldn't share copies of `queries` between invocations. (Maybe that's a weird thing to worry about, but having written compilers it's something that crosses my mind.) Also, I might also worry about that state leaking in other ways. Mutating Python's default arguments comes to mind.

Moreover, some languages (like Elixir in this post) don't even give you the option of having semi-protected local mutable state like the above. (Yes, I know about the process dictionary. No, I don't want to use that—my PR would be rejected immediately by all my coworkers if I did that.)

The point is, this feels like something that shouldn't necessitate having mutable state anywhere, so here's how to get to that nice, pure functional solution without sacrificing readability with tons of boilerplate.

recursive6• 2 years ago

Thanks for your response.

The benefit of immutability is that values can never change "out from under you". But there seems to be a counter-point, which I haven't seen articulated much. In cases where object reference identities are meaningful, all references to an object are always current. Think video game characters running around in an arena interacting with each other.

I think immutability is a great tool for domains where it's helpful. That's the same position I have regarding mutability. I think the mutable Date object in javascript was a mistake. I also think it's hard to compete with the simplicity of a recursive `walk()` function that mutates the AST nodes and query queue in place.

Maybe I'll see the light when I manage to comprehend what a monad is.

Boilerplate busting in functional languages

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