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Today's Topics:
1. How would I increment or otherwise change a value in a
record with ?Simon-ness? (Michael Litchard)
2. Re: How would I increment or otherwise change a value in a
record with ?Simon-ness?
(Sumit Sahrawat, Maths & Computing, IIT (BHU))
----------------------------------------------------------------------
Message: 1
Date: Wed, 5 Aug 2015 14:22:59 -0700
From: Michael Litchard <mich...@schmong.org>
To: "beginners@haskell.org" <beginners@haskell.org>
Subject: [Haskell-beginners] How would I increment or otherwise change
a value in a record with ?Simon-ness?
Message-ID:
<CAEzeKYpSBiCF9x3W2wR-6u9Ymm5vmAs=+lguwrsppstbhzs...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
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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Message: 2
Date: Thu, 6 Aug 2015 02:54:52 +0530
From: "Sumit Sahrawat, Maths & Computing, IIT (BHU)"
<sumit.sahrawat.ap...@iitbhu.ac.in>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <beginners@haskell.org>,
Haskell-cafe Cafe <haskell-c...@haskell.org>
Subject: Re: [Haskell-beginners] How would I increment or otherwise
change a value in a record with ?Simon-ness?
Message-ID:
<cajbew8pi5kxys7ms+cxlwktg20m00wfvvw3ncuxge+szhyx...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
More suitable for the haskell-cafe. Routing.
On 6 August 2015 at 02:52, Michael Litchard <mich...@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
>
>
> _______________________________________________
> Beginners mailing list
> Beginners@haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>
--
Regards
Sumit Sahrawat
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