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Today's Topics:
1. Re: Literate Haskell - capturing output (Martin Drautzburg)
2. Re: f . g or f g or f $ g? (Ertugrul S?ylemez)
3. Re: f . g or f g or f $ g? (Brent Yorgey)
4. Re: f . g or f g or f $ g? (Denis Kasak)
5. Re: Using Parsec to parse Haskell Language (Henk-Jan van Tuyl)
----------------------------------------------------------------------
Message: 1
Date: Fri, 1 Feb 2013 22:23:01 +0100
From: Martin Drautzburg <[email protected]>
Subject: Re: [Haskell-beginners] Literate Haskell - capturing output
To: [email protected]
Message-ID: <[email protected]>
Content-Type: Text/Plain; charset="iso-8859-15"
On Thursday, 31. January 2013 15:25:56 Rustom Mody wrote:
> If you are ok with emacs,
> emacs -> orgmode -> babel may be worth a consider
> http://orgmode.org/worg/org-contrib/babel/intro.html
> http://www.jstatsoft.org/v46/i03/paper
Yes, I'm okay with emacs and I use org-mode a lot. Can you point be to an
example of using org-mode with haskell? I've only seen that as a way to add
program output to a documentation, but will I still end up with a runnable
haskell program?
--
Martin
------------------------------
Message: 2
Date: Fri, 1 Feb 2013 23:02:39 +0100
From: Ertugrul S?ylemez <[email protected]>
Subject: Re: [Haskell-beginners] f . g or f g or f $ g?
To: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset="us-ascii"
Martin Drautzburg <[email protected]> wrote:
> I frequently get confused over f . g vs f g. I do understand the
> following
Out of the three choices "f . g", "f g" and "f $ g", only the dot
operator has a special meaning. The others are the same thing.
Remember the function composition operator from math? It's the circle
operator. That's how you should read the dot. In fact, if you use
Emacs, you can make it display the dot that way. The haskell-mode has
this built in.
(f . g) x = f (g x)
There is also something interesting to note about application:
f $ x = ($) f x
($) f x = f x
>From that follows:
($) f = f
And from that follows:
($) = id
Indeed:
id :: a -> a
($) :: (a -> b) -> (a -> b)
($) is really just id with a more special type.
I hope this helps. =)
Greets,
Ertugrul
--
Not to be or to be and (not to be or to be and (not to be or to be and
(not to be or to be and ... that is the list monad.
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Message: 3
Date: Fri, 1 Feb 2013 17:04:44 -0500
From: Brent Yorgey <[email protected]>
Subject: Re: [Haskell-beginners] f . g or f g or f $ g?
To: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset=us-ascii
On Fri, Feb 01, 2013 at 08:42:56PM +0100, Martin Drautzburg wrote:
>
> Is there a way to easily remember when to use f.g and when to use f g without
> having to do this type algebra.
No.
After enough practice, however, this sort of type algebra becomes
second nature, and no longer requires much conscious effort.
-Brent
------------------------------
Message: 4
Date: Fri, 1 Feb 2013 23:12:23 +0100
From: Denis Kasak <[email protected]>
Subject: Re: [Haskell-beginners] f . g or f g or f $ g?
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Message-ID:
<canjrnzcn1a1zjnzhchjyfbapd4ihetr6edaydewabhmo7m1...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
On 1 February 2013 20:42, Martin Drautzburg <[email protected]>wrote:
> Hello all
>
> I frequently get confused over f . g vs f g. I do understand the following
>
> With:
>
> g :: a->b
> f :: b ->c
>
> f.g :: a->c
>
> However
>
> f g
>
> is a type error, because then f would have to accept a function (a->b) as
> its
> first parameter, but it only accepts a b.
>
> Is there a way to easily remember when to use f.g and when to use f g
> without
> having to do this type algebra.
>
If you're familiar with the analogous operations in mathematics (function
composition and function application), it should be easy to reason about.
Function application is the act of "calling" the function: passing it an
argument and making it "return" a result. In math, we write function
application with parentheses, i.e.
cos(pi)
which "returns" (or /has value of/) -1. The equivalent in Haskell would be
written simply as
cos pi
Function composition on the other hand is the act of combining two
functions (e.g. f1 and f2) so that the resultant function performs both
operations in sequence. You can think of it as combining the functions in a
pipeline so that the output of the first is passed as an argument to the
second. Again, in mathematics, we'd write the composition of f1 and f2 as
f2 ? f1
In Haskell, we would write the same as
f2 . f1
This is a good mnemonic for remembering the role of (.) since the dot looks
a bit like the small circle used for functional composition in mathematics.
--
Denis Kasak
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Message: 5
Date: Fri, 01 Feb 2013 23:55:56 +0100
From: "Henk-Jan van Tuyl" <[email protected]>
Subject: Re: [Haskell-beginners] Using Parsec to parse Haskell
Language
To: [email protected], "Sean Cormican" <[email protected]>
Message-ID: <[email protected]>
Content-Type: text/plain; charset=iso-8859-15; format=flowed;
delsp=yes
On Thu, 31 Jan 2013 17:40:49 +0100, Sean Cormican
<[email protected]> wrote:
> I'm working on a project in which I am attempting to write a parser for
> Haskell within Haskell itself using Parsec.
Do you know about the haskell-src package[0]? As you didn't receive any
answer to your questions yet, I suggest you ask them on Haskell cafe, as
there are more experts there.
Regards,
Henk-Jan van Tuyl
[0] http://hackage.haskell.org/package/haskell-src
--
http://Van.Tuyl.eu/
http://members.chello.nl/hjgtuyl/tourdemonad.html
Haskell programming
--
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