Is there an equivalent to `also` in arrow for things like logging? Currently doing something like ...

# arrow

[Cody Mikol]

Is there an equivalent to also in arrow for things like logging?

Currently doing something like

foo()
.mapLeft { it.also { logger.error("Something terrible has happened!") } }
.map { it.also { logger.success("Something good has happened!") } }

[Marius Kotsbak]

flatTap?: https://arrow-kt.io/docs/0.10/arrow/typeclasses/monad/#flattap-formerly-effectm

[Cody Mikol]

That sounds cool, I see it was removed though

[Marius Kotsbak]

No, only effectM? You need the latest version though I think.

[Cody Mikol]

Those docs you linked are v0.10, I couldn’t find anything like that in the v0.13.2 docs

[Cody Mikol]

But .tap .tapLeft .flatTap sound like they are exactly what I’m looking for

[Marius Kotsbak]

Yeah, we had them as local utility functions here. Could remove them when we upgrade.

[Cody Mikol]

Came up with this

package com.openanywhere.oautils.arrow

import arrow.core.Either

inline fun <L, R> Either<L, R>.tap(fn: () -> Unit) = this.map { it.also { fn() } }

inline fun <L, R> Either<L, R>.tapLeft(fn: () -> Unit) = this.mapLeft { it.also { fn() } }

[Cody Mikol]

not sure if flatTap makes sense as flatMap expects an either return type given the right path

[Cody Mikol]

Could do something cool like

[Cody Mikol]

inline fun <L, R> Either<L, R>.tapFold(lFn: () -> Unit, rFn: () -> Unit) = this
    .tap { rFn() }
    .tapLeft { lFn() }

[Marius Kotsbak]

If you find something new you could try submitting a PR.

[Cody Mikol]

foo().tapFold(:logErr,:logSuccess)

[Cody Mikol]

If this is something that the arrow team is interested in, I’d make a PR

[Marius Kotsbak]

I don't know, but it's possible, if there is something new that is missing.