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Today's Topics:
1. Re: How to understand the type "ShowS"? (Rustom Mody)
2. Re: Overwriting wxFrame::ProcessEvent in wxHaskell
(Henk-Jan van Tuyl)
3. finding the exact instance of a function causing an error
(Nathan H?sken)
4. Re: finding the exact instance of a function causing an
error (Julian Arni)
5. :trace seems not to work in ghci (Nathan H?sken)
----------------------------------------------------------------------
Message: 1
Date: Wed, 25 Sep 2013 17:30:45 +0530
From: Rustom Mody <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] How to understand the type "ShowS"?
Message-ID:
<CAJ+TeocxosisSaSAq1NXn3ubA9ZX7FGX=e9j-ukt6pzb-br...@mail.gmail.com>
Content-Type: text/plain; charset="iso-8859-1"
On Tue, Sep 24, 2013 at 3:45 PM, yi lu <[email protected]>wrote:
> Prelude> :i ShowS
> type ShowS = String -> String -- Defined in `GHC.Show'
>
> It is a type of a function? I cannot understand this type, and don't know
> how to create functions of this type.
>
> And this function "shows"
>
> Prelude> :i shows
> shows :: Show a => a -> ShowS -- Defined in `GHC.Show'
>
> I don't know how this function works.
>
>
Just attempting to give an imperative programmer's motivation for shows.
Look at the picture at start of http://en.wikipedia.org/wiki/Linked_list
which corresponds to the Haskell list
a = [12,19,37]
and 'a' would be a pointer pointing to the 12-node.
Now if we wanted to add something -- say 8 -- before 12, its a simple
operation:
Make a new node containing 8,
point it to the 12-node
Assign a to this new node [Remember we are in imperative-land!!]
This is a simple or constant-time operation
However adding something to the end is harder: we have to start from a and
walk down the list till the end and then mutate it to the new 8-node -- an
O(n) operation where n is the length of the list.
How can we have an O(1) add_to_end operation?
Well in imperative land there are many approaches, one of which is that for
every list like a, we also store an a_end pointing to the last element of a
and then mutate that. This converts an O(n) operation into an O(1)
operation at the cost of a mere extra pointer.
shows is the same idea in FP-land!
The haskell list a = [12,19,37] is as you surely know
a = 12 : (19 : (37: []))
Replace the [] all the way in with an x and to make it valid lambda-bind
it; ie
a = \ x -> 12 : (19 : (37 : x))
a :: [Int] -> [Int]
If the Int were Char that would be the Shows type
And that lambda-exp basically constitutes your 'pointer-to-the-end'
In particular with normal lists
[12,19,37] ++ some_list
needs 3 steps to walk down the first list
However instead of [12,19,37] if we have the lambda-exp 'a' above we just
need to do
a some_list
and we have append being done by a single beta reduction step! Voila!
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Message: 2
Date: Wed, 25 Sep 2013 14:10:20 +0200
From: "Henk-Jan van Tuyl" <[email protected]>
To: "Haskell Beginners Mailinglist" <[email protected]>, Nathan
H?sken <[email protected]>
Subject: Re: [Haskell-beginners] Overwriting wxFrame::ProcessEvent in
wxHaskell
Message-ID: <[email protected]>
Content-Type: text/plain; charset=utf-8; format=flowed; delsp=yes
On Wed, 04 Sep 2013 10:48:10 +0200, Nathan H?sken
<[email protected]> wrote:
> I want that in wxHaskell to! But I do not know if and how I can
> overwrite the ProcessEvent function of wxFrame in wxHaskell ... anyone
> knows if there is a way?
Maybe you can use the blog article "How does wxHaskell event handling work
? part 1"
http://wewantarock.wordpress.com/2011/06/17/how-does-wxhaskell-event-handling-work-part-1/
It seems that there is no part 2 of this story.
Regards,
Henk-Jan van Tuyl
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Haskell programming
--
------------------------------
Message: 3
Date: Wed, 25 Sep 2013 23:57:38 +0200
From: Nathan H?sken <[email protected]>
To: Haskell Beginners Mailinglist <[email protected]>
Subject: [Haskell-beginners] finding the exact instance of a function
causing an error
Message-ID: <[email protected]>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Hey,
I have a program, that, when I run it fails with: Main: Prelude.tail:
empty list
Can I somehow find out which exat instance of tail causes this error?
I tried compiling with -prof and running with +RTX -xc, which gives me:
*** Exception (reporting due to +RTS -xc): (THUNK_2_0), stack trace:
GHC.List.CAF
--> evaluated by: SYSTEM.SYSTEM
*** Exception (reporting due to +RTS -xc): (THUNK_2_0), stack trace:
GHC.List.CAF
*** Exception (reporting due to +RTS -xc): (THUNK_2_0), stack trace:
GHC.List.CAF
Does not really help me ...
Thanks!
Nathan
------------------------------
Message: 4
Date: Wed, 25 Sep 2013 18:05:26 -0400
From: Julian Arni <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] finding the exact instance of a
function causing an error
Message-ID:
<CANct4CRxs9zB=pgg7tg0fzxsjorb_zkypdju2avganyg7pn...@mail.gmail.com>
Content-Type: text/plain; charset="iso-8859-1"
Try running the program with the ghci debugger and
-fbreak-on-error<http://www.haskell.org/ghc/docs/7.6.2/html/users_guide/ghci-debugger.html#ghci-debugger-exceptions>
.
On Wed, Sep 25, 2013 at 5:57 PM, Nathan H?sken <[email protected]>wrote:
> Hey,
>
> I have a program, that, when I run it fails with: Main: Prelude.tail:
> empty list
>
> Can I somehow find out which exat instance of tail causes this error?
> I tried compiling with -prof and running with +RTX -xc, which gives me:
>
> *** Exception (reporting due to +RTS -xc): (THUNK_2_0), stack trace:
> GHC.List.CAF
> --> evaluated by: SYSTEM.SYSTEM
> *** Exception (reporting due to +RTS -xc): (THUNK_2_0), stack trace:
> GHC.List.CAF
> *** Exception (reporting due to +RTS -xc): (THUNK_2_0), stack trace:
> GHC.List.CAF
>
> Does not really help me ...
>
> Thanks!
> Nathan
> ______________________________**_________________
> Beginners mailing list
> [email protected]
> http://www.haskell.org/**mailman/listinfo/beginners<http://www.haskell.org/mailman/listinfo/beginners>
>
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Message: 5
Date: Thu, 26 Sep 2013 12:35:51 +0200
From: Nathan H?sken <[email protected]>
To: Haskell Beginners Mailinglist <[email protected]>
Subject: [Haskell-beginners] :trace seems not to work in ghci
Message-ID: <[email protected]>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Hey,
I have a program I want to debug in ghci because it crashes with an
exception. So I do:
Prelude> :l Main
Ok, modules loaded: Main, PlotDisplay, PlotDiagram, ForexData.
Prelude Main> :set -fbreak-on-exception
Prelude Main> :trace main
(...)
Stopped at <exception thrown>
_exception :: e = _
[<exception thrown>] Prelude Main> :hist
Empty history. Perhaps you forgot to use :trace?
[<exception thrown>] Prelude Main> :back
no more logged breakpoints
[<exception thrown>] Prelude Main> :force _exception
_exception = GHC.Exception.SomeException
(GHC.Exception.ErrorCall "Prelude.tail: empty list")
What could be the reason why :hist/:back/:trace are not working?
Thanks!
Nathan
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