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by tickettotranai 251 days ago
No I believe he's basically formally correct.

You need to be able to "wrap" values and then also "wrap" functions in the way you expect. That's literally it.

Btw, the list monad example is stupid imo and borderline misleading. The promise/nullable/Either examples are better. you "wrap" a function by putting it as the only value in a list, and "map" pretty much acts as your function wrapper, but technically this you need to jump through a couple hoops to make it monadic, and I'm just not sure the metaphor is helpful here
2 comments
dllthomas 251 days ago
For some monads the "wrap" gets awfully metaphorical (for State it's a function that produces the value and updated state, for Const there is no value, etc) but I don't think that's actually a problem, just a thing to be aware of. There is certainly no expectation that you can actually get your hands on the thing.

A bigger issue is that you're missing a piece. If you can "wrap" values and "wrap" functions such that they operate on wrapped values, you (probably) have a functor. To be a monad you also need to have the ability to turn multiple layers of wrapping into one layer of wrapping. For lists, that's "flatten".

I said "probably" above because there are rules these pieces need to follow to behave well. They're pretty simple but I don't think we need to dig into them at this level of discussion.

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internet_points 251 days ago
Agree about the list monad. I have never used it in actual code. I always felt it was so arbitrary, and "implicit", in the sense that if you haven't learnt beforehand that it makes "joins", then it's certainly not obvious that it should work that way

    λ> do { a <- [1,2]; pure a; }
    [1,2]
    -- oh so it returns the list unchanged
    λ> do { a <- [1,2]; b <- [3,4]; pure a; }
    [1,1,2,2]
    -- no what is this spooky action at a distance
    λ> do { a <- [1,2]; b <- [3,4]; pure [a]; }
    [[1],[1],[2],[2]]
    -- ...
    λ> do { a <- [1,2]; b <- [3,4]; c <- []; pure [a]; }
    []
    λ> do { a <- [1,2]; b <- [3,4]; c <- [5]; pure [a,b,c]; }
    [[1,3,5],[1,4,5],[2,3,5],[2,4,5]]
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