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Today's Topics:
1. Functions in "where" block. (Venu Chakravorty)
2. Re: Functions in "where" block. (Kim-Ee Yeoh)
3. Re: tutorials on space complexity? (Dimitri DeFigueiredo)
----------------------------------------------------------------------
Message: 1
Date: Wed, 11 Jun 2014 21:54:42 -0400 (EDT)
From: Venu Chakravorty <[email protected]>
To: [email protected]
Subject: [Haskell-beginners] Functions in "where" block.
Message-ID: <[email protected]>
Content-Type: text/plain; charset="us-ascii"
Hello everyone,
I can't figure out why the following function does not work:
myFun :: Integer -> Integer
myFun x = op x + ep x
where op x = 99
ep x = 1 -- squawks here
Yeah it's a stupid function, but I was just trying to declare two
functions in the "where" block. I expected
this to work and always return 100.
This is the error I get while compiling:
==================
Prelude> :l fun.hs
[1 of 1] Compiling Main ( fun.hs, interpreted )
fun.hs:4:15: parse error on input `='
Failed, modules loaded: none.
==================
Could someone please tell me where I am going wrong?
Thanks in advance.
Regards,
Venu Chakravorty.
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Message: 2
Date: Thu, 12 Jun 2014 09:21:56 +0700
From: Kim-Ee Yeoh <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Functions in "where" block.
Message-ID:
<CAPY+ZdQV-dKPEz=hnybwwuzazsogqbmheweii+vad9mjtre...@mail.gmail.com>
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On Thu, Jun 12, 2014 at 8:54 AM, Venu Chakravorty <[email protected]> wrote:
> fun.hs:4:15: parse error on input `='
>
The problem wasn't obvious until I copy-pasted it into an editor:
You have tabs instead of spaces for the line defining ep.
The rule of thumb is that your editor must expand all tabs into matching
spaces for whitespace-sensitive languages like haskell. Or else you'll
waste time with this kind of parsing errors.
-- Kim-Ee
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Message: 3
Date: Thu, 12 Jun 2014 01:32:39 -0300
From: Dimitri DeFigueiredo <[email protected]>
To: [email protected]
Subject: Re: [Haskell-beginners] tutorials on space complexity?
Message-ID: <[email protected]>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Thanks for the links. The guidance on your blog post is exactly the kind
of analysis I was looking for.
Dimitri
Em 10/06/14 06:20, Heinrich Apfelmus escreveu:
> Dimitri DeFigueiredo wrote:
>> Are there any good tutorials on understanding space complexity for
>> haskell programs?
>>
>> My current approach of "waiting for it to crash" by being out of
>> memory, doesn't really seem like good engineering practice. However,
>> I have not found a source that gives me any proactive insight into
>> what should be avoided. Most of what I have read only helps to solve
>> the problem "after the fact". How do we design programs that avoid
>> those problems from the beginning? Any pointers?
>
> Lazy evaluation makes it difficult to reason about space usage -- it's
> not compositional anymore. However, I have found the following
> technique, dubbed "space invariants", to be very helpful:
>
> http://apfelmus.nfshost.com/blog/2013/08/21-space-invariants.html
>
> The main idea is that because it is impossible to trace lazy
> evaluation in detail, we have to use invariants. In particular, we can
> attach bounds on space usage to semantic meaning. Example:
>
> "If this container with 5 elements is in WHNF, then it will use only
> as much space as the 5 elements."
>
> (This invariant seems banal, but the point is that lazy evaluation
> does not preserve it.)
>
> (WHNF means "weak head normal form", i.e. the expression has been
> evaluated to the outermost constructor.)
>
>
> Best regards,
> Heinrich Apfelmus
>
> --
> http://apfelmus.nfshost.com
>
> _______________________________________________
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