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Today's Topics:
1. Re: Get rid of Maybes in complex types (Sylvain Henry)
2. Re: Get rid of Maybes in complex types (Imants Cekusins)
3. Re: Get rid of Maybes in complex types (Baa)
----------------------------------------------------------------------
Message: 1
Date: Thu, 6 Jul 2017 15:09:17 +0200
From: Sylvain Henry <sylv...@haskus.fr>
To: beginners@haskell.org
Subject: Re: [Haskell-beginners] Get rid of Maybes in complex types
Message-ID: <0edaaa31-0241-2a1d-ef98-1967e3b72...@haskus.fr>
Content-Type: text/plain; charset=utf-8; format=flowed
Hi,
You can use something similar to "Trees that grows" in GHC:
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE StandaloneDeriving #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE UndecidableInstances #-}
module Main where
import Data.Maybe
data Checked = Checked deriving (Show)
data Unchecked = Unchecked deriving (Show)
type family F a b :: * where
F Unchecked b = Maybe b
F Checked b = b
-- data types are decorated with a phantom type indicating if they have
been checked
-- in which case "Maybe X" are replaced with "X" (see F above)
data A c = A
{ a1 :: F c (B c)
}
data B c = B
{ b1 :: F c (C c)
}
data C c = C
{ c1 :: F c Int
}
deriving instance Show (F c (B c)) => Show (A c)
deriving instance Show (F c (C c)) => Show (B c)
deriving instance Show (F c Int) => Show (C c)
class Checkable a where
check :: a Unchecked -> a Checked
instance Checkable A where
check (A mb) = A (check (fromJust mb))
instance Checkable B where
check (B mc) = B (check (fromJust mc))
instance Checkable C where
check (C mi) = C (fromJust mi)
main :: IO ()
main = do
let
a :: A Unchecked
a = A (Just (B (Just (C (Just 10)))))
a' :: A Checked
a' = check a
print a
print a'
$> ./Test
A {a1 = Just (B {b1 = Just (C {c1 = Just 10})})}
A {a1 = B {b1 = C {c1 = 10}}}
Cheers,
Sylvain
On 06/07/2017 10:12, Baa wrote:
> Hello Dear List!
>
> Consider, I retrieve from external source some data. Internally it's
> represented as some complex type with `Maybe` fields, even more, some
> of fields are record types and have `Maybe` fields too. They are
> Maybe's because some information in this data can be missing (user
> error or it not very valuable and can be skipped):
>
> data A = A {
> a1 :: Maybe B
> ... }
> data B = B {
> b1 :: Maybe C
> ... }
>
> I retrieve it from network, files, i.e. external world, then I validate
> it, report errors of some missing fields, fix another one (which can be
> fixed, for example, replace Nothing with `Just default_value` or even I
> can fix `Just wrong` to `Just right`, etc, etc). After all of this, I
> know that I have "clean" data, so all my complex types now have `Just
> right_value` fields. But I need to process them as optional, with
> possible Nothing case! To avoid it I must create copies of `A`, `B`,
> etc, where `a1`, `b1` will be `B`, `C`, not `Maybe B`, `Maybe C`. Sure,
> it's not a case.
>
> After processing and filtering, I create, for example, some resulting
> objects:
>
> data Result {
> a :: A -- not Maybe!
> ... }
>
> And even more: `a::A` in `Result` (I know it, after filtering) will not
> contain Nothings, only `Just right_values`s.
>
> But each function which consumes `A` must do something with possible
> Nothing values even after filtering and fixing of `A`s.
>
> I have, for example, function:
>
> createResults :: [A] -> [Result]
> createResults alst =
> ...
> case of (a1 theA) ->
> Just right_value -> ...
> Nothing ->
> logError
> undefined -- can not happen
>
> Fun here is: that it happens (I found bug in my filtering
> code with this `undefined`). But now I thought about it: what is the
> idiomatic way to solve such situation? When you need to have:
>
> - COMPLEX type WITH Maybes
> - the same type WITHOUT Maybes
>
> Alternative is to keep this Maybes to the very end of processing, what I
> don't like. Or to have types copies, which is more terrible, sure.
>
> PS. I threw IOs away to show only the crux of the problem.
>
> ---
> Cheers,
> Paul
> _______________________________________________
> Beginners mailing list
> Beginners@haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
------------------------------
Message: 2
Date: Thu, 6 Jul 2017 16:37:05 +0300
From: Imants Cekusins <ima...@gmail.com>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <beginners@haskell.org>
Subject: Re: [Haskell-beginners] Get rid of Maybes in complex types
Message-ID:
<cap1qinzudv6aw4sxh0dvijvdbns8mitzc7ivkea7fdf_udz...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
> "Trees that grows"
this (type families), or Tagged
http://hackage.haskell.org/package/tagged-0.8.5/docs/Data-Tagged.html
data Checked = Checked
Tagged Checked a
On 6 July 2017 at 16:09, Sylvain Henry <sylv...@haskus.fr> wrote:
> Hi,
>
> You can use something similar to "Trees that grows" in GHC:
>
> {-# LANGUAGE TypeFamilies #-}
> {-# LANGUAGE StandaloneDeriving #-}
> {-# LANGUAGE FlexibleContexts #-}
> {-# LANGUAGE UndecidableInstances #-}
>
> module Main where
>
> import Data.Maybe
>
> data Checked = Checked deriving (Show)
> data Unchecked = Unchecked deriving (Show)
>
> type family F a b :: * where
> F Unchecked b = Maybe b
> F Checked b = b
>
> -- data types are decorated with a phantom type indicating if they have
> been checked
> -- in which case "Maybe X" are replaced with "X" (see F above)
> data A c = A
> { a1 :: F c (B c)
> }
>
> data B c = B
> { b1 :: F c (C c)
> }
>
> data C c = C
> { c1 :: F c Int
> }
>
> deriving instance Show (F c (B c)) => Show (A c)
> deriving instance Show (F c (C c)) => Show (B c)
> deriving instance Show (F c Int) => Show (C c)
>
> class Checkable a where
> check :: a Unchecked -> a Checked
>
> instance Checkable A where
> check (A mb) = A (check (fromJust mb))
>
> instance Checkable B where
> check (B mc) = B (check (fromJust mc))
>
> instance Checkable C where
> check (C mi) = C (fromJust mi)
>
> main :: IO ()
> main = do
> let
> a :: A Unchecked
> a = A (Just (B (Just (C (Just 10)))))
>
> a' :: A Checked
> a' = check a
> print a
> print a'
>
>
> $> ./Test
> A {a1 = Just (B {b1 = Just (C {c1 = Just 10})})}
> A {a1 = B {b1 = C {c1 = 10}}}
>
>
> Cheers,
> Sylvain
>
>
>
> On 06/07/2017 10:12, Baa wrote:
>
>> Hello Dear List!
>>
>> Consider, I retrieve from external source some data. Internally it's
>> represented as some complex type with `Maybe` fields, even more, some
>> of fields are record types and have `Maybe` fields too. They are
>> Maybe's because some information in this data can be missing (user
>> error or it not very valuable and can be skipped):
>>
>> data A = A {
>> a1 :: Maybe B
>> ... }
>> data B = B {
>> b1 :: Maybe C
>> ... }
>>
>> I retrieve it from network, files, i.e. external world, then I validate
>> it, report errors of some missing fields, fix another one (which can be
>> fixed, for example, replace Nothing with `Just default_value` or even I
>> can fix `Just wrong` to `Just right`, etc, etc). After all of this, I
>> know that I have "clean" data, so all my complex types now have `Just
>> right_value` fields. But I need to process them as optional, with
>> possible Nothing case! To avoid it I must create copies of `A`, `B`,
>> etc, where `a1`, `b1` will be `B`, `C`, not `Maybe B`, `Maybe C`. Sure,
>> it's not a case.
>>
>> After processing and filtering, I create, for example, some resulting
>> objects:
>>
>> data Result {
>> a :: A -- not Maybe!
>> ... }
>>
>> And even more: `a::A` in `Result` (I know it, after filtering) will not
>> contain Nothings, only `Just right_values`s.
>>
>> But each function which consumes `A` must do something with possible
>> Nothing values even after filtering and fixing of `A`s.
>>
>> I have, for example, function:
>>
>> createResults :: [A] -> [Result]
>> createResults alst =
>> ...
>> case of (a1 theA) ->
>> Just right_value -> ...
>> Nothing ->
>> logError
>> undefined -- can not happen
>>
>> Fun here is: that it happens (I found bug in my filtering
>> code with this `undefined`). But now I thought about it: what is the
>> idiomatic way to solve such situation? When you need to have:
>>
>> - COMPLEX type WITH Maybes
>> - the same type WITHOUT Maybes
>>
>> Alternative is to keep this Maybes to the very end of processing, what I
>> don't like. Or to have types copies, which is more terrible, sure.
>>
>> PS. I threw IOs away to show only the crux of the problem.
>>
>> ---
>> Cheers,
>> Paul
>> _______________________________________________
>> Beginners mailing list
>> Beginners@haskell.org
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>
>
> _______________________________________________
> Beginners mailing list
> Beginners@haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
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------------------------------
Message: 3
Date: Thu, 6 Jul 2017 17:41:25 +0300
From: Baa <aqua...@gmail.com>
To: beginners@haskell.org
Subject: Re: [Haskell-beginners] Get rid of Maybes in complex types
Message-ID: <20170706174125.0efbfa06@Pavel>
Content-Type: text/plain; charset=UTF-8
Hello, Sylvain. Hmm, it's very interesting. Funny is that I already
have tagged items but tags are run-time values, not compile time
(types) :-) but this is a different.
Problem was to make:
A-with-maybes -> A-without-maybes
Phantom type as flag and "clearing" of Maybe with
family-type-with-maybes -> family-type-without-maybes
looks promisingly. Another advantage, as I understand, is that I
continue to use Just as a constructor for `a1` value, without to wrap it
in something else, right?
This seems to be a solution.
Thank you and all others for your answers!!
---
Best regards,
Paul
В Thu, 6 Jul 2017 15:09:17 +0200
Sylvain Henry <sylv...@haskus.fr> wrote:
> Hi,
>
> You can use something similar to "Trees that grows" in GHC:
>
> {-# LANGUAGE TypeFamilies #-}
> {-# LANGUAGE StandaloneDeriving #-}
> {-# LANGUAGE FlexibleContexts #-}
> {-# LANGUAGE UndecidableInstances #-}
>
> module Main where
>
> import Data.Maybe
>
> data Checked = Checked deriving (Show)
> data Unchecked = Unchecked deriving (Show)
>
> type family F a b :: * where
> F Unchecked b = Maybe b
> F Checked b = b
>
> -- data types are decorated with a phantom type indicating if they
> have been checked
> -- in which case "Maybe X" are replaced with "X" (see F above)
> data A c = A
> { a1 :: F c (B c)
> }
>
> data B c = B
> { b1 :: F c (C c)
> }
>
> data C c = C
> { c1 :: F c Int
> }
>
> deriving instance Show (F c (B c)) => Show (A c)
> deriving instance Show (F c (C c)) => Show (B c)
> deriving instance Show (F c Int) => Show (C c)
>
> class Checkable a where
> check :: a Unchecked -> a Checked
>
> instance Checkable A where
> check (A mb) = A (check (fromJust mb))
>
> instance Checkable B where
> check (B mc) = B (check (fromJust mc))
>
> instance Checkable C where
> check (C mi) = C (fromJust mi)
>
> main :: IO ()
> main = do
> let
> a :: A Unchecked
> a = A (Just (B (Just (C (Just 10)))))
>
> a' :: A Checked
> a' = check a
> print a
> print a'
>
>
> $> ./Test
> A {a1 = Just (B {b1 = Just (C {c1 = Just 10})})}
> A {a1 = B {b1 = C {c1 = 10}}}
>
>
> Cheers,
> Sylvain
>
>
> On 06/07/2017 10:12, Baa wrote:
> > Hello Dear List!
> >
> > Consider, I retrieve from external source some data. Internally it's
> > represented as some complex type with `Maybe` fields, even more,
> > some of fields are record types and have `Maybe` fields too. They
> > are Maybe's because some information in this data can be missing
> > (user error or it not very valuable and can be skipped):
> >
> > data A = A {
> > a1 :: Maybe B
> > ... }
> > data B = B {
> > b1 :: Maybe C
> > ... }
> >
> > I retrieve it from network, files, i.e. external world, then I
> > validate it, report errors of some missing fields, fix another one
> > (which can be fixed, for example, replace Nothing with `Just
> > default_value` or even I can fix `Just wrong` to `Just right`, etc,
> > etc). After all of this, I know that I have "clean" data, so all my
> > complex types now have `Just right_value` fields. But I need to
> > process them as optional, with possible Nothing case! To avoid it I
> > must create copies of `A`, `B`, etc, where `a1`, `b1` will be `B`,
> > `C`, not `Maybe B`, `Maybe C`. Sure, it's not a case.
> >
> > After processing and filtering, I create, for example, some
> > resulting objects:
> >
> > data Result {
> > a :: A -- not Maybe!
> > ... }
> >
> > And even more: `a::A` in `Result` (I know it, after filtering) will
> > not contain Nothings, only `Just right_values`s.
> >
> > But each function which consumes `A` must do something with possible
> > Nothing values even after filtering and fixing of `A`s.
> >
> > I have, for example, function:
> >
> > createResults :: [A] -> [Result]
> > createResults alst =
> > ...
> > case of (a1 theA) ->
> > Just right_value -> ...
> > Nothing ->
> > logError
> > undefined -- can not happen
> >
> > Fun here is: that it happens (I found bug in my filtering
> > code with this `undefined`). But now I thought about it: what is the
> > idiomatic way to solve such situation? When you need to have:
> >
> > - COMPLEX type WITH Maybes
> > - the same type WITHOUT Maybes
> >
> > Alternative is to keep this Maybes to the very end of processing,
> > what I don't like. Or to have types copies, which is more terrible,
> > sure.
> >
> > PS. I threw IOs away to show only the crux of the problem.
> >
> > ---
> > Cheers,
> > Paul
> > _______________________________________________
> > Beginners mailing list
> > Beginners@haskell.org
> > http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
> _______________________________________________
> Beginners mailing list
> Beginners@haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
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