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<https://downloads.haskell.org/~ghc/8.0.1/docs/html/users_guide/glasgow_exts.html#custom-errors>

This could be used in propellor to improve compile time errors.

For example, a RevertableProperty is sometimes used where only a regular
Property is accepted. In this case, the error could suggest that the user
apply `setupRevertableProperty` to extract the setup side of the RevertableProperty.

> The `custom-error-types` branch has this implemented quite nicely.
> Unfortunately, it ran into a ghc bug <https://gitlab.haskell.org/ghc/ghc/issues/16894>

[[!tag user/joey]]