While discussing whether `decoded` in `pipes-binary` should be an
`Iso'` a `Getter'` or a `Lens'` [1], I started thinking what would be
the advantages of each one from the parsing point of view.
The obvious advantage of the `Iso'` would be that it's automatically
reversible using `from`[2], so you get `encoded` for free. But it turns
out it's not truly reversible when facing errors, `lens-family` doesn't
include the `from` function and it's in the end not that obvious to use
for end users.
Using `Lens'`es for `decoded` and `encoded` raises the question that if
we are considering the lenses irreversible, why not just use
`Getter'`s? They wouldn't violate the lens laws and could be composed
correctly with other lenses, folds, traversals..., when applicable.
After all, if we have `Getter`s that transform `Producer`s as we need,
what's the use of the *setter* part of a `Producer` lens?
Well, it turns out that `zoom` can't use `Getter`s [3], but I fail to
understand how it uses the *setter* part, changing what the *setter*
does, doesn't seem to affect `zoom`. There is also `magnify` which
works like `zoom` with `Getter`s but it can't operate on the state
monad [4].
So I was thinking why not include a version of `zoom` in `pipes-parse`
that works with `Getter`s, using `mapStateT` for example [5], and base
all pipes parsing on `Getter`s by default? What would be the
disadvantages of this approach?
[1] https://github.com/k0001/pipes-binary/issues/12
[2]
http://hackage.haskell.org/package/lens-4.0.1/docs/Control-Lens-Iso.html#v:from
[3]
http://hackage.haskell.org/package/lens-4.0.1/docs/Control-Lens-Zoom.html#v:zoom
[4]
http://hackage.haskell.org/package/lens-4.0.1/docs/Control-Lens-Zoom.html#v:magnify
[5]
http://hackage.haskell.org/package/transformers-0.3.0.0/docs/Control-Monad-Trans-State-Strict.html#v:mapStateT
-- Danny Navarro
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