Add replaceRight and replaceLeft functions

[mrcjkb]

• [x] By raising this pull request you confirm you are licensing your contribution under all licenses that apply to this project (see LICENSE) and that you have no patents covering your contribution.

• [x] If you care, my PR preferences are at https://github.com/ndmitchell/neil#contributions, but they're all guidelines, and I'm not too fussy - you don't have to read them.

I find the replaceLeft function useful for "translating" error handling from a library's domain to my own domain (to encapsulate the library's domain).

Example (pseudo code):

• External library:

  gitInfo :: Either GitHashException GitInfo

• Encapsulation of GitHashException:

  doSomethingSpecial :: Either MyAppError MyResultType
  doSomethingSpecial = replaceLeft SpecialError res
  where 
   res = doSomeThingSpecialWith <$> gitInfo

[ndmitchell]

Thanks for the patch. I wonder how useful these functions are? Wouldn't you usually want to replace the left by wrapping the underlying error, so that the original error message is still available? I note that replaceRight is already available as $> - which I'm generally a fan of more specialised functions, things like $> are pretty universal, so a little less concerning.

[mrcjkb]

I wonder how useful these functions are? Wouldn't you usually want to replace the left by wrapping the underlying error, so that the original error message is still available?

In most cases, I would agree. In some cases, I am replacing an error that has a no-arg constructor with my own error that has a no-arg constructor. There is a direct mapping between SpecialError and a specific error constructor, (so no information is lost by not wrapping the error) and the sole purpose is encapsulation of the libraries I am using. I could use SpecialError a with a generic argument to encapsulate the library's error from my API, but in this case it would just lead to more code and a less simple API without any benefits.

Another use case would be if I want to log the original Left and then replace it with a simpler error message that is later printed to stderr in a command line app.

I personally have not had any use for replaceRight. I just added it because the module has a Right function for each Left function and vice versa.

[ndmitchell]

Thanks for the contribution. Alas, I think these functions probably don't being in extra just yet, for 4 interlinked reasons:

• They don't save that much code - mapLeft . const isn't that much longer.
• It's not clear exactly what these functions do based on their name - often replace is used with string/list replacement, not the replacing on inner components of structures.
• It's not clear that using them for the purpose of manipulating error messages is a good idea. Haskell programmers have diverse opinions about how errors should be handled, but dropping parts of the error seems dangerous, and I think we'd need a solid set of examples where it is the right thing before I'd be convinced it's a good default to follow.
• They don't have significant use just yet. I'd recommend defining them in your program as a local definition, and then see how useful they are in practice. If they turn out to show up everywhere, that might be a sign they are useful in extra.