Consider Unsafe or Partial for `from*`

[toastal]

A recent version of Data.Either moved fromRight and fromLeft to no longer be be partial but to provide a default. Perhaps these should be a Data.Either.Partial.from* version as can be seen with: purescript/purescript-strings#128

[garyb]

The better option now is to use either (unsafeCrashWith "...") identity, where ... is some kind of explanation of what it is or why it went wrong.

We could perhaps add them back under an Either.Unsafe module, but I think it would be better if they included a string still, so there's some clue to what went wrong in the inconceivable case that they do actually fail. 😉

[hdgarrood]

I would prefer not to re-add them; I don’t think they’re useful often enough to justify a spot here, especially since writing fromRight (crashWith "...") is only one more function application.

[toastal]

That's a smart option too! I wasn't sure the correct route would be. But I can update the strings lib once the message is settled.

[natefaubion]

The better option now is to use either (unsafeCrashWith "...") identity, where ... is some kind of explanation of what it is or why it went wrong.

Let’s not forget which language we are programming in 😅 😜 This will totally type check and crash immediately.

[hdgarrood]

Oh, haha, oops. So I guess we'd need to do something like either (\_ -> unsafeCrashWith "...") identity. The total version of fromRight is not suitable for this at all then, I guess you'd have to do something like (_ $ unit) <<< fromRight (\_ -> unsafeCrashWith "..") <<< map const.

Maybe we want lazy versions of the form fromRight' :: forall a b. (Unit -> b) -> Either a b -> b?

[garyb]

Ah yeah, I used either exactly so it could be (\_ -> ....) in that branch, rather than suggesting fromRight (crashWith ..., but then didn't write it out properly 😉. But yeah, adding the lazy fromLeft / fromRight would also work and perhaps be worthwhile.

[toastal]

Assuming this is a common thing (which wouldn't surprise me given that it's in the library already), what should the message say? "Invalid RegExp: input_string_here"?

[hdgarrood]

We should probably reuse the error from the Left constructor, so actually I suppose either unsafeCrashWith identity should work.

[toastal]

Seems reasonable. I'll have a look after my shower.

[JordanMartinez]

Since Maybe has a lazy version of fromMaybe: fromMaybe', should this library have the same for fromLeft and fromRight, too?

[kl0tl]

Should we consider an unsafeFromRight :: Either String a -> a function as an alias for either unsafeCrashWith identity?

Otherwise is there anything left to do now that we have fromLeft' :: forall a b. (Unit -> a) -> Either a b -> a and fromRight' :: forall a b. (Unit -> b) -> Either a b -> b or can we close this issue?

[hdgarrood]

I think I would prefer to close the issue; the fromLeft' and fromRight' functions definitely ought to help fill this gap, and more API surface is a liability. I think arguably we shouldn't be encouraging use of unsafe functions by making them too comfortable to reach; that's a principle we've followed in the core libraries fairly consistently.

[JordanMartinez]

I'm in favor of closing this, too.