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Comment by sfvisser | Hacker News Reader

Nice writeup.

Although I really like the fact that lenses (from the `lens` package) can be composed using regular function composition, instead of the more generalized category composition like `fclabels` uses, I still think it's weird that the direction of composition is flipped.

I reads more like OO, but a lot less like idiomatic Haskell.

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I also think it's a bit weird to flip direction of composition, but there are a couple of advantages --

1. It keeps the "imperative" feel, so that you can write

    person.address.zipCode

rather than

    zipCode.address.person

2. There's an argument that mathematical functions are "back to front" and that it would be more natural to write functions left to right, i.e. (x)f rather than f(x), so that the function composition f.g is "apply f then apply g" rather than "apply g then apply f" which is where we are at the moment.

Let's just go ahead and steal (<) and (>) for directional function composition everywhere. I'd rather just write `lt` and `gt`.

    x & f > g > h === h < g < f $ x

We already have (<<<) and (>>>) so you're nearly there :)

Those always feel comical to use to me. It's like having a 747 landing strip to help guide a duck in for a safe landing.

Yes they are rather big :)

> There's an argument that mathematical functions are "back to front" and that it would be more natural to write functions left to right

Totally agreed, but until programming languages adopt "x f = ..." for a definition of f then I'd prefer to stick to composing the traditional way round.

I strongly prefer the regular composition order. But I also use fclabels (FTW!)

Changing the direction of composition makes no sense.

Composition order is the weirdest thing about Control.Lens.

"Changing the direction of composition makes no sense."

I also can't stand all these operators of the lens package. It feels like reading Perl code.

The only annoying thing about all the other lens packages is the need for hiding '.' from Prelude.

I thought that function composition (.) arises from the category instance for (->)?

Actually on checking it seems that Prelude re-exports (.) from Data.Function. I'm sure I read somewhere that these had been unified, was I wrong?

I believe the argument against unifying is that you get very hard to decipher error messages.

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