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Today's Topics:
1. Initialize a data type in (Writer (Endo a)) monoid
(Dmitriy Matrosov)
2. Re: Initialize a data type in (Writer (Endo a)) monoid
(Brent Yorgey)
----------------------------------------------------------------------
Message: 1
Date: Mon, 13 Jan 2014 21:54:55 +0400
From: Dmitriy Matrosov <[email protected]>
To: [email protected]
Subject: [Haskell-beginners] Initialize a data type in (Writer (Endo
a)) monoid
Message-ID: <[email protected]>
Content-Type: text/plain; charset=US-ASCII
Hi.
I have the following data type
> import Data.Monoid
> import Control.Monad.Writer
> data Graph = Graph {graphTitle :: String, graphPoints :: [Int]}
> deriving (Show)
> emptyGraph :: Graph
> emptyGraph = Graph {graphTitle = "", graphPoints = []}
Then i want to initialize it in Writer monad using (Dual (Endo Graph))
as monoid:
> type GraphM t = WriterT (Dual (Endo Graph)) t
>
> myGraph :: Monad t => GraphM t [Int]
> myGraph = do
> tell (setTitle "ab")
> tell (setTitle "ABCD")
> tell (modifyPoints (1 :))
> tell (modifyPoints (2 :))
> return [1, 2]
and then i can use it e.g. like
> getGraph :: Monad t => (GraphM t a) -> t Graph
> getGraph m = do
> g <- execWriterT m
> return (appEndo (getDual g) emptyGraph)
>
> printGraph :: IO ()
> printGraph = getGraph myGraph >>= print
And to make this work i need two functions implemented for each
record of Graph:
get :: Graph -> a
set :: a -> Graph -> Graph
I can define per-record instances of them, like
> setTitle :: String -> Dual (Endo Graph)
> setTitle x = Dual . Endo $ \g -> g{graphTitle = x}
>
> modifyPoints :: ([Int] -> [Int]) -> Dual (Endo Graph)
> modifyPoints f = Dual . Endo $ \g -> let xs = graphPoints g
> in g{graphPoints = f xs}
but if Graph has many records, for most of them these 'set' functions ('get'
functions i'll have from records) will look very similar. Is there a way how i
can define all of them "in one row", i.e. using some generic 'set'
implementation to which i should only pass e.g. record name or smth else?
I.e. so, that above myGraph will look like
..
tell(set r x)
..
where r is record name (or smth else referencing particulat record) and x is
value.
------------------------------
Message: 2
Date: Mon, 13 Jan 2014 14:07:27 -0500
From: Brent Yorgey <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Initialize a data type in (Writer
(Endo a)) monoid
Message-ID: <[email protected]>
Content-Type: text/plain; charset=us-ascii
On Mon, Jan 13, 2014 at 09:54:55PM +0400, Dmitriy Matrosov wrote:
>
> but if Graph has many records, for most of them these 'set' functions ('get'
> functions i'll have from records) will look very similar. Is there a way how i
> can define all of them "in one row", i.e. using some generic 'set'
> implementation to which i should only pass e.g. record name or smth else?
> I.e. so, that above myGraph will look like
>
> ..
> tell(set r x)
> ..
Yes, you can do this with the 'lens' package. The package is big and
complicated, but here's all you need to know:
0) import Control.Lens
1) Name your fields with underscores:
> data Graph = Graph { _graphTitle :: String, _graphPoints :: [Int] }
> deriving (Show)
2) Add the line
> $(makeLenses ''Graph)
after the definition of Graph (also be sure the TemplateHaskell
extension is enabled)
3) Now you can use the 'view' and 'set' functions (or their infix
equivalents, (^.) and (.~)), like so:
..
tell (set graphTitle x)
..
or
tell (graphTitle .~ x)
makeLenses generated a special lens called 'graphTitle' from the field
name '_graphTitle'.
Of course the above doesn't actually typecheck since (set graphTitle x)
is a function but you need a Dual Endo, but you can easily make your
own custom set function that adds the Dual Endo wrapper, etc.
-Brent
------------------------------
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