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Today's Topics:
1. Get rid of Maybes in complex types (Baa)
2. Re: Get rid of Maybes in complex types (Imants Cekusins)
3. Incomplete patterns (Jona Ekenberg)
4. Re: Incomplete patterns (Sylvain Henry)
----------------------------------------------------------------------
Message: 1
Date: Thu, 6 Jul 2017 11:12:55 +0300
From: Baa <aqua...@gmail.com>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <beginners@haskell.org>
Subject: [Haskell-beginners] Get rid of Maybes in complex types
Message-ID: <20170706111255.1d6695b8@Pavel>
Content-Type: text/plain; charset=US-ASCII
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
------------------------------
Message: 2
Date: Thu, 6 Jul 2017 11:24:39 +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:
<CAP1qinZ2ydUksEJDkFK=qtfeh45_jl_g70wbc-2mm3yrz_i...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
Identity
http://hackage.haskell.org/package/mtl-2.2.1/docs/Control-Monad-Identity.html
may work:
data A m = A {
a1 :: m B
}
data B m = B {
b1 :: m C
... }
m: Maybe or Identity
- any good?
On 6 July 2017 at 11:12, Baa <aqua...@gmail.com> 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
>
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Message: 3
Date: Thu, 6 Jul 2017 11:12:04 +0200
From: Jona Ekenberg <saik...@gmail.com>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <beginners@haskell.org>
Subject: [Haskell-beginners] Incomplete patterns
Message-ID:
<calveeueldexztzoflx-wy6fvwn-oc5bgxs6wxftg7qdbsvx...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
Hello,
I am currently going through the CIS194 spring 13 course, since it has been
recommended by several haskellions. I find the exercises extremely
valuable, and the reading material is succinct (in a good way).
I just finished the exercises for lesson two, and while everything works, I
get this warning:
Lektion02.hs:(38,1)-(44,21): warning: [-Wincomplete-patterns] …
Pattern match(es) are non-exhaustive
In an equation for ‘insert’:
Patterns not matched:
(LogMessage _ _ _) (Node _ (LogMessage _ _ _) _)
I understand that there is a pattern I'm not covering, but I can't really
figure out which one. Here is my function:
insert :: LogMessage -> MessageTree -> MessageTree
insert (Unknown _) mt = mt
insert m Leaf = Node Leaf m Leaf
insert m@(LogMessage _ t _) (Node left value@(LogMessage _ t' _) right)
| t <= t' = Node (insert m left) value right
| t > t' = Node left value (insert m right)
insert m@(LogMessage _ _ _) (Node left (Unknown _) right)
= Node left m right
(here's a pastebin with intact indentation: http://lpaste.net/356716)
I am not sure on how I should progress to find out which pattern I'm
missing.
Any help is appreciated.
Kind regards,
Jona
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Message: 4
Date: Thu, 6 Jul 2017 11:21:35 +0200
From: Sylvain Henry <sylv...@haskus.fr>
To: beginners@haskell.org
Subject: Re: [Haskell-beginners] Incomplete patterns
Message-ID: <203988f4-5c4c-e7f1-5d5a-ed5555ec5...@haskus.fr>
Content-Type: text/plain; charset="utf-8"; Format="flowed"
Hi,
In the following code:
insert m@(LogMessage _ t _) (Node left value@(LogMessage _ t' _) right)
| t <= t' = Node (insert m left) value right
| t > t' = Node left value (insert m right)
GHC can't detect that both guards (t <= t') and (t > t') cover all
cases. The usual way to deal with this is to use "otherwise" (which is
always true):
insert m@(LogMessage _ t _) (Node left value@(LogMessage _ t' _) right)
| t <= t' = Node (insert m left) value right
| otherwise = Node left value (insert m right)
Regards,
Sylvain
On 06/07/2017 11:12, Jona Ekenberg wrote:
> Hello,
>
> I am currently going through the CIS194 spring 13 course, since it has
> been recommended by several haskellions. I find the exercises
> extremely valuable, and the reading material is succinct (in a good way).
>
> I just finished the exercises for lesson two, and while everything
> works, I get this warning:
> Lektion02.hs:(38,1)-(44,21): warning: [-Wincomplete-patterns] …
> Pattern match(es) are non-exhaustive
> In an equation for ‘insert’:
> Patterns not matched:
> (LogMessage _ _ _) (Node _ (LogMessage _ _ _) _)
>
> I understand that there is a pattern I'm not covering, but I can't
> really figure out which one. Here is my function:
> insert :: LogMessage -> MessageTree -> MessageTree
> insert (Unknown _) mt = mt
> insert m Leaf = Node Leaf m Leaf
> insert m@(LogMessage _ t _) (Node left value@(LogMessage _ t' _) right)
> | t <= t' = Node (insert m left) value right
> | t > t' = Node left value (insert m right)
> insert m@(LogMessage _ _ _) (Node left (Unknown _) right)
> = Node left m right
>
> (here's a pastebin with intact indentation: http://lpaste.net/356716)
>
> I am not sure on how I should progress to find out which pattern I'm
> missing.
> Any help is appreciated.
>
> Kind regards,
> Jona
>
>
> _______________________________________________
> Beginners mailing list
> Beginners@haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
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