By: Tom Sydney Kerckhove <syd@cs-syd.eu>
Add autodocodec-http-api-data for form-urlencoded data A type with a HasObjectCodec instance could not be posted as application/x-www-form-urlencoded without a second, hand-written description of its fields. http-api-data's own generic derivation is no substitute: it derives names from Haskell record fields via a label modifier, so it would be a third independent spelling of the same field names rather than a shared one. This is autodocodec-servant-multipart's interpreter retargeted at Web.FormUrlEncoded.Form, which is likewise a flat map from text keys to text values. A Form's HashMap Text [Text] models repeated keys natively, so ArrayOfCodec fits it better than it fits multipart's flat list of inputs, and unlike the multipart spec every corpus type round-trips, Example included. Three places where copying multipart verbatim would have been wrong: * Form derives Semigroup from HashMap, whose union is left-biased and so would silently drop the second form's values under a shared key, where multipart's mappend concatenates. Hence unionForm. * Booleans encode lower case, to match toUrlPiece @Bool and the HTML form convention, so that a value survives http-api-data's own parser. The decoder still accepts either spelling. * A field is absent exactly when its key is absent, which is lossless. dropEmptyFormValues is exported separately, for callers who need a shell's unset variable to read as absence rather than as the empty string.