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

[Rein Henrichs]

This sort of thing is (relatively) nicely solved by lenses with
typeclasses, e.g.,

class HasName a where
  name :: Lens' a String

Then, with the appropriate lenses,

person ^. name
person & name .~ "Ben"
person1 ^. name
person1 & name .~ "Jerry"


On Wed, Aug 5, 2015 at 2:53 PM Michael Litchard <michael at schmong.org> wrote:

> 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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>>> Beginners at haskell.org
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>>>
>>>
>>
>>
>> --
>> Regards
>>
>> Sumit Sahrawat
>>
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>>
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