See the 24 days of PureScript 2016 post.

The Warn type class

There is a type class in Prim called Warn - which, like Fail is indexed by a Symbol. When the compiler solves a Warn constraint it will trivially solve the instance and print out the message as a user defined warning.

Deprecation example

One possible use case for this is custom deprecation warnings. You start with a library that exports a function notBad:

notBad :: Int
notBad = 21

But now you decide there is something better; you want to deprecate notBad in favour of this new function:

notBad :: Warn (Text "`notBad` is deprecated. Prefer `better` instead.") => Int
notBad = 21

better :: Int
better = 42

Now when someone uses notBad, they see the following compiler warning:

  A custom warning occurred while solving type class constraints:

    `notBad` is deprecated. Prefer `better` instead.