More suitable for the haskell-cafe. Routing. On 6 August 2015 at 02:52, Michael Litchard wrote:

The below code is from this tutorial http://dev.stephendiehl.com/hask/

it illustrates very well how to operate on values from records with "Simon-ness" (illustrated below). What I am struggling with is how to modify values inside records with "Simon-ness", say incrementing "age". I keep thinking it has to do with the way Label is defined with the constructor Get. Could I add another constructor Put?

{-# LANGUAGE DataKinds #-}{-# LANGUAGE KindSignatures #-}{-# LANGUAGE MultiParamTypeClasses #-}{-# LANGUAGE FunctionalDependencies #-}{-# LANGUAGE FlexibleInstances #-}{-# LANGUAGE FlexibleContexts #-}{-# LANGUAGE StandaloneDeriving #-}{-# LANGUAGE ExistentialQuantification #-}{-# LANGUAGE ConstraintKinds #-}

import GHC.TypeLits newtype Field (n :: Symbol) v = Field { unField :: v } deriving Show data Person1 = Person1 { _age :: Field "age" Int , _name :: Field "name" String } data Person2 = Person2 { _age' :: Field "age" Int , _name' :: Field "name" String , _lib' :: Field "lib" String } deriving instance Show Person1deriving instance Show Person2 data Label (l :: Symbol) = Get class Has a l b | a l -> b where from :: a -> Label l -> b instance Has Person1 "age" Int where from (Person1 a _) _ = unField a instance Has Person1 "name" String where from (Person1 _ a) _ = unField a instance Has Person2 "age" Int where from (Person2 a _ _) _ = unField a instance Has Person2 "name" String where from (Person2 _ a _) _ = unField a

age :: Has a "age" b => a -> b age pnt = from pnt (Get :: Label "age")

name :: Has a "name" b => a -> b name pnt = from pnt (Get :: Label "name") -- Parameterized constraint kind for "Simon-ness" of a record.type Simon a = (Has a "name" String, Has a "age" Int)

spj :: Person1 spj = Person1 (Field 56) (Field "Simon Peyton Jones")

smarlow :: Person2 smarlow = Person2 (Field 38) (Field "Simon Marlow") (Field "rts")

catNames :: (Simon a, Simon b) => a -> b -> String catNames a b = name a ++ name b

addAges :: (Simon a, Simon b) => a -> b -> Int addAges a b = age a + age b

names :: String names = name smarlow ++ "," ++ name spj-- "Simon Marlow,Simon Peyton Jones"

ages :: Int ages = age spj + age smarlow-- 94

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-- Regards Sumit Sahrawat