A functional version of withShownDefault

[pbrisbin]

I have a custom type for bytes that I use as an option with a better syntax: something like "128m" -> Bytes 128 $ Just M. withShownDefault is perfect for getting default: 128m to appear in the documentation, but its ergonomics were not what I expected. Ultimately, I had to write:

withShownDefault (Bytes 128 $ Just M) (showBytes $ Bytes 128 $ Just M) $ setting
    [ help "Run restylers with --memory"
    , option
    , name "memory"
    , metavar "NUMBER<b|k|m|g>"
    , reader $ eitherReader readBytes
    ]

I have to specify the default value twice, which could lead to bugs. I know I could extract with let or where, of course, but that's not always convenient.

When I first saw withShownDefault I assumed it would work like this:

withShownDefault :: (a -> String) -> a -> Parser a -> Parser a
withShownDefault = undefined

withDefault :: a -> Parser a -> Parser a
withDefault = withShownDefault show

Then I could simply do,

withShownDefault showBytes (Bytes 128 $ Just M) $ setting
    [ help "Run restylers with --memory"
    , option
    , name "memory"
    , metavar "NUMBER<b|k|m|g>"
    , reader $ eitherReader readBytes
    ]

I'm interested in adding a function that works this way, but with the name withShownDefault taken, I'm not sure what to call it.

withShownByDefault :: (a -> String) -> a -> Parser a -> Parser a
withShownByDefault f a = withShownDefault (f a) a

?

[NorfairKing]

I think I prefer your version and we should do that, name it withShownDefault and do a major version break.
You can get the old behaviour back with const.
PR welcome.

[pbrisbin]

I prefer your version and we should do that

Awesome, should I also change valueWithShown so it's functional in the same way?

[NorfairKing]

I prefer your version and we should do that

Awesome, should I also change valueWithShown so it's functional in the same way?

Yes please.
Don't forget a version bump and changelog.
(The date in the changelog is not hugely important.)

[pbrisbin]

Opened as #17.

[NorfairKing]

Merged manually and released.