Type Level "Application" Operator

[Oleg Grenrus]

To make it clear:

type level `.` won’t work as an type synonym, as it’s application isn’t saturated.

{-# LANGUAGE TypeOperators #-}
type (:.:) f g x = f (g x)
infixr 9 :.:

type App = Maybe :.: []

fails to compile with following errors (for a reason):

    • The type synonym ‘:.:’ should have 3 arguments, but has been given 2
    • In the type synonym declaration for ‘App’

> On 02 Nov 2016, at 16:24, Edward Kmett <ekmett at gmail.com> wrote:
> 
> +1, but the operator you're looking for in App there would actually be a type level version of (.).
> 
> type App a = ExceptT Err $ ReaderT Config $ LogT Text $ IO a
> 
> type App = ExceptT Err . ReaderT Config . LogT Text . IO
> 
> which would need
> 
> type (.) f g x = f (g x)
> infixr 9 .
> 
> to parse
> 
> -Edward
> 
> On Tue, Nov 1, 2016 at 7:13 PM, Elliot Cameron <eacameron at gmail.com> wrote:
> Folks,
> 
> Has there been a discussion about adding a type-level operator "$" that just mimics "$" at the value level?
> 
> type f $ x = f x
> infixr 0 $
> 
> Things like monad transformer stacks would look more "stack-like" with this:
> 
> type App = ExceptT Err $ ReaderT Config $ LogT Text IO
> 
> Elliot Cameron
> 
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