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

[Michael Litchard]

I noticed the mail got archived, but I have yet to see it in my mail queue.
Is it just me? Or did something go wrong with distribution?

On Wed, Aug 5, 2015 at 2:24 PM, Sumit Sahrawat, Maths & Computing, IIT
(BHU) <sumit.sahrawat.apm13 at iitbhu.ac.in> wrote:

> More suitable for the haskell-cafe. Routing.
>
> On 6 August 2015 at 02:52, Michael Litchard <michael at schmong.org> 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
>
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