nicuveo

Summary

The code in this gist tries to find a way to have pseudo "multi methods" in C++ by extending a visitor interface, hence the title: extending library visitor for user-defined types with macros. The idea is this: you want to implement the Visitor pattern over a class hierarchy defined in a library. But users of the library might have their own classes, extending the hierarchy, forcing your visitor to know about them. My proposal: a small macros that extend the visitor

nicuveo

While the previous gist documented my attempt at finding a working solution, this one provides a small working example.

Here, the library code declares classes A and B both inheriting form a base class Base. A Visitor is introduced, and both A and B have a apply method that calls the appropriate visitor method.

The client introduces a new class C that also inherits from base, and which is able to use a Visitor-derived class on any Base* pointer, be it a A, a B or a C under the hood.

nicuveo

{-# LANGUAGE ConstraintKinds       #-}
{-# LANGUAGE DataKinds             #-}
{-# LANGUAGE FlexibleContexts      #-}
{-# LANGUAGE FlexibleInstances     #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE ParallelListComp      #-}
{-# LANGUAGE TypeFamilies          #-}
{-# LANGUAGE TypeOperators         #-}
{-# LANGUAGE UndecidableInstances  #-}

nicuveo

{-# LANGUAGE DataKinds         #-}
{-# LANGUAGE FlexibleContexts  #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE TypeFamilies      #-}
{-# LANGUAGE TypeOperators     #-}


import           Data.Proxy
import           GHC.TypeLits

nicuveo

{-# LANGUAGE FlexibleContexts       #-}
{-# LANGUAGE FlexibleInstances      #-}
{-# LANGUAGE FunctionalDependencies #-}
{-# LANGUAGE UndecidableInstances   #-}


-- no data kinds, pure type level
-- let's start by defining booleans and numbers

data True

nicuveo

{"type":"http-log","timestamp":"2020-03-31T00:05:40.203+0000","level":"error","detail":{"operation":{"user_vars":{"x-hasura-role":"admin"},"error":{"internal":{"statement":"DO $$ DECLARE  r RECORD;  BEGIN    FOR r IN (SELECT routine_name FROM information_schema.routines WHERE specific_schema = 'hdb_views' ORDER BY routine_name) LOOP      EXECUTE 'DROP FUNCTION hdb_views.' || quote_ident(r.routine_name) || '() CASCADE';    END LOOP;  END $$; ","prepared":false,"error":{"exec_status":"FatalError","hint":null,"message":"cannot execute DROP FUNCTION in a read-only transaction","status_code":"25006","description":null},"arguments":[]},"path":"$.args[0].args","error":"postgres query error","code":"unexpected"},"request_id":"51d1a76d-8688-4905-a913-f1be94bc8df5","response_size":566,"query":{"args":[{"args":{"cascade":false,"check_metadata_consistency":null,"sql":"SELECT\n  COALESCE(Json_agg(Row_to_json(info)), '[]' :: json) AS tables\nFROM (\n  SELECT\n    pgn.nspname as table_schema,\n    pgc.relname as table_name,

nicuveo

{-# LANGUAGE AllowAmbiguousTypes #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeApplications    #-}

import           Data.Proxy
import           Data.Typeable

contains :: forall a b. (Typeable a, Typeable b) => Bool
contains = go typeB
  where typeA = typeRep (Proxy :: Proxy a)

nicuveo

{-# LANGUAGE AllowAmbiguousTypes  #-}
{-# LANGUAGE DataKinds            #-}
{-# LANGUAGE PolyKinds            #-}
{-# LANGUAGE TypeFamilies         #-}
{-# LANGUAGE TypeOperators        #-}
{-# LANGUAGE UndecidableInstances #-}

import           Data.Kind      (Type)
import           Data.Type.Bool

nicuveo

all:
	ghc --version
	ghc -Wredundant-constraints -Werror -O0 Main.hs
	ghc -Wredundant-constraints -Werror -O2 Main.hs

nicuveo

{-# LANGUAGE CPP                 #-}
{-# LANGUAGE DataKinds           #-}
{-# LANGUAGE DeriveDataTypeable  #-}
{-# LANGUAGE FlexibleContexts    #-}
{-# LANGUAGE GADTs               #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE StandaloneDeriving  #-}
{-# LANGUAGE TypeApplications    #-}
{-# LANGUAGE TypeFamilies        #-}