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

[Sumit Sahrawat, Maths & Computing, IIT (BHU)]

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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