Documentation

The qqLiteral function takes two arguments. The first is a function which parses values of the type we’re interested in, which should return a Right value in case of success, or a Left value with an error message in case of failure. The second is a Name, which must refer to the same function passed as the first argument. It is recommended to use Template Haskell quoting to provide the second argument; see below.

The resulting QuasiQuoter applies the quoted string to the parsing function at compile time, and if parsing succeeds, an expression equivalent to fromRight (parse str) is spliced into the program. If parsing fails, this causes an error at compile time.

As long as the parsing function isn't lying about being pure, this should always be safe, in that it should never result in an error at runtime.

For example, suppose we wanted to be able to define literals for the URI type in network-uri. The function parseURI from network-uri has type String -> Maybe URI, so we first need to define an Either version:

eitherParseURI :: String -> Either String URI
eitherParseURI str =
  maybe (Left ("Failed to parse URI: " ++ str)) Right (parseURI str)

We can then use the qqLiteral function to create our QuasiQuoter. Note that we use a single-quote character to quote eitherParseURI in order to provide the Name argument; this will require the TemplateHaskell extension to be enabled.

uri :: QuasiQuoter
uri = qqLiteral eitherParseURI 'eitherParseURI

And it can be used as follows:

exampleDotCom :: URI
exampleDotCom = [uri|http://example.com/lol|]