The purpose of the Reader monad is to avoid threading arguments through multiple functions in order to only get them where they are needed.
One of...
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Very nice functional concept. But I have small concern if the whole typing of that has/or not a big sense. I mean we really use here a HOF and the whole composition
h . g . f
result is a function withdeps
argumentFor the second, I see some benefit, but not see doing it like below as a bad thing (yes deps are passed explicitly, but is it any issue?)
Not get me wrong, just thinking if the whole additional piping and typing is beneficial here.
Well, you are just writing a bunch of functions returning
Reader
, whether or not you add an explicit type. The point is making the dependencies as implicit as possibile, the key is to put them as a last argument. Note that often people put the dependencies as a first argumentmaking composition more difficult.
all the following
g
s are equivalent (so feel free to choose the style you prefer)I'm used to put deps as the first argument since this lets you partially apply f:
const f = (deps: Dependencies) => (b: boolean): string => (...)
Yes the whole idea has a lot of sense, and I mean adding argument after not before. Thanks for clarifying!
I'd say there'e no magic in here - in FP all functions have to be referentially transparent at the end of the day i.e. all arguments have to be passed directly! What I think is crucial in here is the conceptual "turning inside-out" (or rather "bottom-up", should I say) - instead of passing props/arguments down multiple levels, we pass a function consuming these props (
Reader
) multiple levels up, in an almost implicit way. When the Reader reaches "the surface", we can pass its dependencies directly.Yes I see this in that way also. The whole idea here is in dependency argument in the end and not in the beginning.
In my comment I was more addressing the whole additional idealogy over that, like special typings and using some pipe, ask, chain. I see these as only a fog and complexity over really simple but powerful concept.
There's a benefit in using the monadic interface though: software is written in a uniform style, regardless of the effect.
You can think of
Reader
as an effect, muck likeEither
orTask
.Let's say you have the following snippet
and at some point you must refactor
f
toresult
andpointFreeVersion
must be refactored as well, fortunately you can useReaderEither
's monadic interfaceI was just about to say it: I think main benefit is (ironically) readability - by saying explicitly
Reader<Dependencies, string>
you clearly communicate your intent (assuming others know the concept as well, of course 😄).That's right, you write programs by composing kleisli arrows
A -> M<B>
, for some effectM
.So
Reader
(orReaderEither
,ReaderTaskEither
,Option
,Either
, etc...) is just one of the possible effectsWow, this looks great! 😍
I'm having trouble understanding how the Reader monad can help in more complex examples though.
Take a look at this for instance: (functions with the "2" suffix are the ones that I would write having dependencies as the first parameter)
In particular, notice how:
f
needs to makeb
explicit even if could be implicit (like it is inf2
)g
pipe need to take the result from the previous computation and pass it as the first parameter to the transform functions, while this is implicit ing2
. This leads to less readable code and harder composition. Is there something I'm missing?g
like I did withDependencies & OtherDependencies
?That's exactly what I needed. I saw you just released
chainW
in 2.6.0. Great job thanks! :)Do we have a standard name for
(r: Reader<R,A>) => A
?Yoneda?
Provider<R>
is isomorphic toR
Whoops, my brain is starting to melt, but I like all of this. :)
Wait, wait, I get it now
I know this is an older post, I'm having trouble understanding how this fixes the original issue.
That's true after the refactor as well. Dependencies of
f
are part ofg
andh
signatures. The way the example is written, the problem is compounded wheng
has a dependency, because nowf
depends on parameters ofg
as well. They could be distinct types, but dependencies ofg
must be a superset of dependencies off
, e.g.I understand the desire to curry them so that we can pipe things together, but we haven't lessened the pain of refactoring here at all.
Even if
g
did not directly callf
but was instead piped to it, would it not be better suited to have a flavor offlow
orpipe
that would automatically coalesce the dependencies?Great article!
I'd love to see another article about Writer using fp-ts.