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Parametric type for an intermediate value when combining functions

example with the folling problem(s):

function buildNRender has an ambigious intermediate parametric type
the type of buildNRender has no parametric type g involved at all
- hence, it does not make sense to have buildNRender as a member of class Builder

code

module Main where
 
import qualified Data.Word as W
import qualified Octetable as Oct
 
main :: IO ()
main = 
    do
        print (buildNRender "123")
 
data MyData = MyData Integer
 
data Construction model = Contains model | Error String
    deriving Show
 
class Builder g where
    build :: String -> (Construction g)
    render :: (Construction g) -> [W.Word8]
    buildNRender :: String -> [W.Word8]
    buildNRender = render . build
 
instance Builder MyData where
    build s = Contains (MyData (read s :: Integer))
    render (Contains (MyData n)) = Oct.toOctets n
    render (Error _) = []

example that solves the problem with extensions

code

{-# LANGUAGE AllowAmbiguousTypes, TypeApplications, ScopedTypeVariables #-}
 
module Main where
 
import qualified Data.Word as W
import qualified Octetable as Oct
 
main :: IO ()
main = 
    do
        print (buildNRender @MyData "123")
 
data MyData = MyData Integer
 
data Construction model = Contains model | Error String
    deriving Show
 
class Builder g where
    build :: String -> (Construction g)
    render :: (Construction g) -> [W.Word8]
 
buildNRender :: forall g . Builder g => String -> [W.Word8]
-- forall g . introduces the Scope of Type Variable g
-- needs extension AllowAmbiguousTypes
buildNRender = render . build @g -- @g is a Type Application
 
instance Builder MyData where
    build s = Contains (MyData (read s :: Integer))
    render (Contains (MyData n)) = Oct.toOctets n
    render (Error _) = []

example that solves the problem without extensions

code

module Main where
 
import qualified Data.Word as W
import qualified Octetable as Oct
 
main :: IO ()
main = 
    do
        print (buildNRender (Proxy :: Proxy MyData) "123")
 
data MyData = MyData Integer
 
data Construction model = Contains model | Error String
    deriving Show
 
class Builder g where
    build :: String -> (Construction g)
    render :: (Construction g) -> [W.Word8]
    buildNRender :: proxy g -> String -> [W.Word8]
    buildNRender p = render . blub p build
        where
            blub :: proxy g -> f (c g) -> f (c g)
            blub _ = id
 
instance Builder MyData where
    build s = Contains (MyData (read s :: Integer))
    render (Contains (MyData n)) = Oct.toOctets n
    render (Error _) = []
 
data Proxy a = Proxy

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