headerContentID is not a valid Lens

[tomjaguarpaw]

It looks like headerContentID is not a valid Lens, because parse failure maps to Nothing, whilst setting Nothing removes the header in question from the list of headers. Thus set l (view l s) s = s is violated. Perhaps consider making it a Traversal' a ContentID instead?

> import Data.MIME
> import Data.IMF.Syntax
> import Data.String
> let l = headerContentID
> let s = Headers [(mk (fromString "Message-ID"), fromString "! unparseable !")]
> import Control.Lens
> set l (view l s) s
Headers []
> s
Headers [("Message-ID","! unparseable !")]

(Using Message-ID in this example because of #75)

purebred-email/src/Data/MIME.hs

Lines 922 to 926 in 84ab531

	headerContentID :: (HasHeaders a) => Lens' a (Maybe ContentID)
	headerContentID = headers . at "Message-ID" . iso (>>= f) (fmap g)
	where
	f = either (const Nothing) Just . parseOnly (parseContentID <* endOfInput)
	g = renderContentID