Test.TypeSpec.Internal.Result

Result type used in constraints inside TypeSpec to propagate type errors.

Documentation

type Result = Either ErrorMessage

When a type level expectation is tested, it might be that compound expectations fail. In order to have a small, precise error message, the type level assertion results are made to have kind Result.

type family Try (e :: Result k) :: k where ...

Equations

Try (OK (d :: k)) = d

Try (FAILED m) = TypeError m

type family DontTry (e :: Result r) :: Constraint where ...

Equations

DontTry (FAILED e) = ()

DontTry (OK okIsNotOk) = TypeError ((Text "You specified this wasn't okay: " :$$: (Text " " :<>: ShowType okIsNotOk)) :$$: Text "... turns out it actually is!")

type FAILED = Left

type OK = Right

A nice name than Left

type family PrependToError (message :: ErrorMessage) (result :: Result a) :: Result a where ...

Equations

PrependToError message (OK x) = OK x

PrependToError message (FAILED otherMessage) = FAILED (message :<>: otherMessage)