Require explicit type application for some type variables.

say-my-name

This is a small, slightly cursed package that lets you require explicit type applications for certain type variables. Blame @taylorfausak for asking if it could be done, but not if it should be done!

To force explicit type annotations on a type variable a, add a new type variable a_ and a constraint MustName a "a" a_:

-- NOTE: requires AllowAmbiguousTypes to define!
sayMyNameId :: forall a_ a. (MustName a "a" a_) => a -> a
sayMyNameId = id

This constraint means "the user must supply a value for a via an explicit type annotation that specifies a_". The string "a" is used to make nice error messages:

λ> sayMyNameId "test"
sayMyNameId "test"

<interactive>:57:1: error:
    • The type parameter `a` must be supplied by an explicit type application, even if it could be inferred.
    • When checking the inferred type
        it :: forall t.
              (Break (TypeError ...) t, Data.String.IsString (SayMyName_ t)) =>
              SayMyName_ t
λ> sayMyNameId "test" :: String
sayMyNameId "test" :: String

<interactive>:58:1: error:
    • The type parameter `a` must be supplied by an explicit type application, even if it could be inferred.
    • In the expression: sayMyNameId "test" :: String
      In an equation for ‘it’: it = sayMyNameId "test" :: String
λ> sayMyNameId ("test" :: String)
sayMyNameId ("test" :: String)

<interactive>:59:1: error:
    • The type parameter `a` must be supplied by an explicit type application, even if it could be inferred.
    • In the expression: sayMyNameId ("test" :: String)
      In an equation for ‘it’: it = sayMyNameId ("test" :: String)
λ> sayMyNameId @String "test"
sayMyNameId @String "test"
"test"

You can also augment the error message with a usage example:

-- NOTE: requires AllowAmbiguousTypes to define!
sayMyNameId :: forall a_ a. (MustNameEx a "a" a_ "sayMyNameId @String \"hello, world!\"") => a -> a
sayMyNameId = id

λ> sayMyNameId "test"

<interactive>:36:1: error:
    • The type parameter `a` must be supplied by an explicit type application, even if it could be inferred. For example: sayMyNameId @String "hello, world!"
    • In the expression: sayMyNameId "test"
      In an equation for ‘it’: it = sayMyNameId "test"