[Haskell-beginners] Dispatch on implemented instances

18 Oct 2017

      Hello, everyone!

I want to call one function (or the same but with a special argument, no 
matter) if a type instantiates some type-class or another - if not. I 
know that it's easy in languages like D, Java, possible C++ - we can 
specialize templates more generic or less generic and to restrict its 
parameters with class/interface implementation/extending (there are 
special keywords/type operators for it, used in generics/templates 
signatures). So, I suppose Haskell can do the same. But as it turned 
out, this is not so easy for Haskell :(

I found these useful links:

  * https://stackoverflow.com/questions/26303353/can-multiple-dispatch-be-achiev...
  * https://wiki.haskell.org/GHC/AdvancedOverlap

So, I wrote something like this (this example is compiling and works as 
I expect, you need `mustache` and `network-uri` packages). Goal is to 
substitute type in special way for HTML template and for text template 
if type implements a special interface (for deterministic substitution). 
Otherwise default substitution will be used. Actually, this code allows 
usage of not only HTML/text templates... OK, here it is:

    {-# LANGUAGE EmptyDataDecls         #-}
    {-# LANGUAGE FlexibleInstances      #-}
    {-# LANGUAGE FunctionalDependencies #-}
    {-# LANGUAGE MultiParamTypeClasses  #-}
    {-# LANGUAGE OverloadedStrings      #-}
    {-# LANGUAGE ScopedTypeVariables    #-}
    {-# LANGUAGE TypeFamilies           #-}
    {-# LANGUAGE UndecidableInstances   #-}

    import           Data.Semigroup      ((<>))
    import qualified Data.Text           as T
    import           Network.URI
    import qualified Text.Mustache.Types as MUT

    type Link = (T.Text, URI) -- caption and URL, for example

    -- |Converts a type to substitutable to Mustache template entity
    based on 'SubTo' template selector.
    -- @sub@ must be ignored in implementation
    class ToMustaches sub a where
       toMustaches :: sub -> a -> SubTo -> MUT.Value

    -- |Substituted to HTML either to text
    data SubTo = ToHtml | ToText

    -- |Deterministic substitution
    data DetSub
    -- |Universal substitution
    data UniSub

    -- |Substitution predicate: it's determined uniquely for type
    class SubPred a sub | a -> sub

    -- TODO rename ToMustaches' (ToMustaches' ?)
    class ToMustaches' a where
       toMustaches' :: a -> SubTo -> MUT.Value
    instance (SubPred a sub, ToMustaches sub a) => ToMustaches' a where
       toMustaches' a to = toMustaches (undefined::sub) a to

    -- See:
    -- *
    https://stackoverflow.com/questions/36913922/how-to-resolve-overlapping-inst...
    -- *
    https://stackoverflow.com/questions/26303353/can-multiple-dispatch-be-achiev...
    instance {-# OVERLAPS #-} (sub ~ UniSub) => SubPred a sub

    -- 'Link' can be substituted deterministically
    instance SubPred Link DetSub
    instance ToMustaches DetSub Link where
       toMustaches _ (_, url)   ToText = MUT.String $ T.pack $ show url
       toMustaches _ (cap, url) ToHtml = MUT.String ("
    (T.pack $ show url) <> "\">" <> cap <> "</a>")

    -- |All other types which does not implements 'ToMustaches' default
    instance transforms them to string w/ 'Show'
    instance Show a => ToMustaches UniSub a where
       toMustaches _ a _ = MUT.String $ T.pack $ show a

    -------------------------------------------------------------------
    main :: IO ()
    main = do
       let
         uri = URI "http:" (Just $ URIAuth "" "wikipedia.org" "") "" "" ""
         lnk = ("wikipedia", uri)::Link
         txt = "Hello world"::T.Text
         n = 444::Int
       print $ toMustaches' lnk ToText
       print $ toMustaches' lnk ToHtml
       print $ toMustaches' txt ToHtml
       print $ toMustaches' n ToText

I don't know am I understand correctly the Haskell solution but this 
looks close to explained in the Haskell Wiki (case 1). But in all cases 
I see problem: always I need not only to implement this special 
interface but to "say" that a type implements special (deterministic) 
substitution, see:

    instance SubPred Link DetSub

This means that clients of this code must "implements" 2 instances: 1) 
of ToMustaches and 2) of SubPred (with DetSub as last type param). This 
is terrible and no such thing  in other languages! So, my question is: 
is a simple way to avoid it?

===

Best regards, Paul

[Haskell-beginners] Dispatch on implemented instances

PY