How would I increment or otherwise change a value in a record with “Simon-ness”

5 Aug 2015

      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

Michael Litchard

Sumit Sahrawat, Maths & Computing, IIT (BHU)

Michael Litchard

Rein Henrichs

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