How to parse a primitive type to a more constrained type? e.g. how to parse an 'IP' from a text field?

[Qqwy]

I am looking to convert e.g. a string or text field that follows certain other rules into a more restricted datatype. For instance, consider the 'server' field in this example https://github.com/kowainik/tomland/blob/main/test/examples/example.toml#L12.

We might want to parse this to a valid IP address datastructure (or fail parsing with a descriptive error). Vice-versa we of course want to be able to pretty-print this IP address back as a string.

So we have a Text -> Maybe IPAddress (or Text -> Either Text IPAddress) for the parsing, and IPAddress -> Text for the prettyprinting.

My hunch is that I'm looking for something that is very close to the signature of dimatch:

Toml.dimatch  :: (b -> Maybe a) -> (a -> b) -> Toml.TomlCodec a -> Toml.TomlCodec b

but with the opposite variance.

Toml.diparse  :: (b -> a) -> (a -> Maybe b) -> Toml.TomlCodec b -> Toml.TomlCodec a

(Of course, using an Either to keep track of the error messages is nicer than using Maybe here).

But maybe there is also a much simpler way that I'm missing? I'm still very new to bidirectional parsing.

[Qqwy]

So there were a few functions I have missed when looking through the documentation before. Conclusion: It is already very possible to do this right now, especially when parsing data from a Text field.

• If the format you want to parse/prettyprint happens to be the same as the Read and Show instances, there is Toml.Codec.Combinator.Custom.read (Re-exported as Toml.read)
• If you have a custom parser/prettyprint function pair, you can use Toml.Codec.Combinator.Custom.textBy. This function essentially has the signature that I was looking for in the question above (specialized to Text), i.e. (a -> Text) -> (Text -> Either Text a) -> Key -> TomlCodec a.
• There is also validate and validateIf but these are less useful in my opinion as they do not follow parse, don't validate. In other words, you can only use them to further refine an a using a validation function whose result is another a.
• In some cases, such as when trying to parse TOML keys (which by themselves are String/Text) to a custom datatype, you might need to drop down to the BiMap layer, most likely using the prism function if only one of the directions might fail. (if both might fail, use the BiMap constructor directly. If neither can fail, use iso). To ensure that your resulting BiMap is a TomlBiMap that you can use for other functions like e.g. Toml.tableMap, wrap the error message (which is probably a Text, depending on what other kind of parsing functionality you use) with the Toml.ArbitraryError constructor.

[Qqwy]

I'm leaving this issue open, because I think that one (or multiple) of these bullet points might be useful to add to the README in the section about translating between TOML and your desired Haskell.

What do you think? If you give the green light, I'd love to contribute a PR for this.

[CGenie]

It would be nice to add this to the docs. I had to dig this issue to see how to write a custom parser for a custom data type.

[meghfossa]

Adding example here, for future readers as I ran into this today

import Text.Megaparsec
import Data.Text (Text)
import Toml (TomlCodec, (.=))
import Toml qualified

-- name: somename 
-- special: [<MyEntity>, <MyEntity>, ...]

data Example = Example
    {
        name :: Text,
        special :: Maybe [MyEntity]
    }
    deriving (Eq, Ord, Show)

exampleCodec :: TomlCodec Example
exampleCodec = Example 
      <$> Toml.diwrap (Toml.text "name") .= name
      <*> Toml.dioptional (Toml.arrayOf myCodec "special") .= special

myCodec :: Toml.TomlBiMap MyEntity Toml.AnyValue
myCodec = Toml._TextBy (toText . show) parseMyEntity

parseMyEntity :: Text -> Either Text MyEntity
parseMyEntity candidate = case runParser myEntityParser "" candidate of
  Left peb -> Left $ toText $ errorBundlePretty peb
  Right rr -> Right rr

myEntityParser :: Parser MyEntity
myEntityParser = .....

[domenkozar]

Here's a codec for uri-bytestring:

uri :: Toml.Key -> TomlCodec (URI.URIRef URI.Absolute)
uri = Toml.match (_URI2ByteString >>> Toml._ByteString)

_URI2ByteString :: Toml.TomlBiMap (URI.URIRef URI.Absolute) ByteString
_URI2ByteString = Toml.BiMap
    { forward  = Right . URI.serializeURIRef'
    , backward = left (Toml.ArbitraryError . show) . URI.parseURI URI.strictURIParserOptions
    }

Maybe we should start tomland-extras?