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Everything Is Just Functions: 1 week with David Beazley and SICP

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389 points kurinikku | 1 comments | 17 Nov 24 15:07 UTC | HN request time: 0s | source
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marvinborner ◴[17 Nov 24 16:10 UTC] No.42164919[source]▶
They give a nice introduction to encoding state as pure functions. In fact, there are many more purely functional encodings for all kinds of data like trees, integers, sum/product types, images, monads, ...

The encodings can be a bit confusing, but really elegant and tiny at the same time. Take for example a functional implementation of the Maybe monad in javascript:

  Nothing = nothing => just => nothing
  Just = v => nothing => just => just(v)
  
  pure = Just
  bind = mx => f => mx(mx)(f)
  
  evalMaybe = maybe => maybe("Nothing")(v => "Just " + v)
  console.log(evalMaybe(bind(Nothing)(n => pure(n + 1)))) // Nothing
  console.log(evalMaybe(bind(Just(42))(n => pure(n + 1)))) // Just 43
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hinkley ◴[17 Nov 24 19:38 UTC] No.42166462[source]▶
[flagged]
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marvinborner ◴[17 Nov 24 21:22 UTC] No.42167412[source]▶
I think it's all right if you're used to the notation. The first two lines are tagged unions and will be recognisable as such if you're familiar with encodings like Scott/Church pairs/lists/numbers. Once you understand the structure, the definition of `bind` becomes obvious, as its two arguments represent the cases "is nothing" and "is just", where in the first case Nothing is returned, and in the second case the function is applied to the value inside the Just.

I think that writing such code, if only for educational purposes, can be really helpful in actually understanding how the state "flows" during the monadic bind/return. Typical monad instantiations of Maybe do not give such deep insight (at least to me).

> Just because you can do a thing doesn’t mean you should.

Of course you should, where would be the fun in that?

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1. salawat ◴[17 Nov 24 21:41 UTC] No.42167560[source]▶
>I think it's all right if you're used to the notation.

Higher mathematics in a nutshell.

>Of course you should, where would be the fun in that?

Also higher mathematics in a nutshell.

Narrator asks: Who should we put in charge of <<thing that will effect people in a tangible way>>?

Not the mathematicians! echo the crowd in unanmity.

Narrator asks: Who will we delegate the task of <<abuse of notation>> to?

The crowd grumbles, arguing amongst themselves whether such a question even warrants an answer. A mathematician stands up, proclaiming "We'll take it!", following up with, "Once you understand the notation involved in my previous statement, you will understand why this outcome is inevitable."

The crowd, seeing the wisdom of not even embarking on that tribulation, assents to the delegation, given the task of undoing the abuse of notation for the legibility of the layperson is also delegated to the aspiring mathematician.

Scene opens on current day...