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Today's Topics:
1. Re: (Integral a) => a vs Integer (Michael Snoyman)
2. Re: (Integral a) => a vs Integer (Dan Ross)
3. safetail problem (Roelof Wobben)
4. Re: safetail problem (Tom Murphy)
5. Re: safetail problem (Roelof Wobben)
6. Re: safetail problem (Tom Murphy)
7. Monads in javascript (Martin Drautzburg)
8. Re: safetail problem (David Place)
9. Re: Monads in javascript (Ertugrul Soeylemez)
10. Re: safetail problem (Ertugrul Soeylemez)
----------------------------------------------------------------------
Message: 1
Date: Tue, 12 Jul 2011 19:57:38 +0300
From: Michael Snoyman <[email protected]>
Subject: Re: [Haskell-beginners] (Integral a) => a vs Integer
To: [email protected]
Cc: [email protected]
Message-ID:
<CAKA2Jg+dJ85=iy26uf3n97c2klzsjagsc-+cakhoanr0+yo...@mail.gmail.com>
Content-Type: text/plain; charset=ISO-8859-1
Precisely.
On Tue, Jul 12, 2011 at 7:44 PM, Dan Ross <[email protected]> wrote:
> Ah. Because Integer is an instance of Integral right?
>
> So using Integer would be more restrictive.
>
> On Tue, 12 Jul 2011 19:22:36 +0300, Michael Snoyman wrote:
>>
>> On Tue, Jul 12, 2011 at 7:20 PM, Dan Ross <[email protected]> wrote:
>>>
>>> Hi all-
>>>
>>> I'm going through Learn You a Haskell for Great Good and I don't
>>> understand
>>> the advantage of using (Integral a)=> a vs just Integer as I show below.
>>>
>>> Could someone explain this to me?
>>>
>>> Thanks,
>>>
>>> Dan
>>>
>>> lucky :: (Integral a) => a -> String
>>> lucky 7 = "LUCKY NUMBER SEVEN!"
>>> lucky x = "Sorry, you're out of luck, pal!"
>>>
>>> lucky :: Integer -> String
>>> lucky 7 = "LUCKY NUMBER SEVEN!"
>>> lucky x = "Sorry, you're out of luck, pal!"
>>
>> Hi Dan,
>>
>> The first version is polymorphic, and will work on *any* instance of
>> Integral. This would allow people to use Int, Int32, Int64, etc. The
>> second version requires the user to pass in specifically an Integer.
>>
>> Michael
>
>
------------------------------
Message: 2
Date: Tue, 12 Jul 2011 12:27:28 -0500
From: Dan Ross <[email protected]>
Subject: Re: [Haskell-beginners] (Integral a) => a vs Integer
To: Michael Snoyman <[email protected]>
Cc: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset=UTF-8; format=flowed
Cool. Thanks.
On Tue, 12 Jul 2011 19:57:38 +0300, Michael Snoyman wrote:
> Precisely.
>
> On Tue, Jul 12, 2011 at 7:44 PM, Dan Ross <[email protected]>
> wrote:
>> Ah. Because Integer is an instance of Integral right?
>>
>> So using Integer would be more restrictive.
>>
>> On Tue, 12 Jul 2011 19:22:36 +0300, Michael Snoyman wrote:
>>>
>>> On Tue, Jul 12, 2011 at 7:20 PM, Dan Ross <[email protected]>
>>> wrote:
>>>>
>>>> Hi all-
>>>>
>>>> I'm going through Learn You a Haskell for Great Good and I don't
>>>> understand
>>>> the advantage of using (Integral a)=> a vs just Integer as I show
>>>> below.
>>>>
>>>> Could someone explain this to me?
>>>>
>>>> Thanks,
>>>>
>>>> Dan
>>>>
>>>> lucky :: (Integral a) => a -> String
>>>> lucky 7 = "LUCKY NUMBER SEVEN!"
>>>> lucky x = "Sorry, you're out of luck, pal!"
>>>>
>>>> lucky :: Integer -> String
>>>> lucky 7 = "LUCKY NUMBER SEVEN!"
>>>> lucky x = "Sorry, you're out of luck, pal!"
>>>
>>> Hi Dan,
>>>
>>> The first version is polymorphic, and will work on *any* instance
>>> of
>>> Integral. This would allow people to use Int, Int32, Int64, etc.
>>> The
>>> second version requires the user to pass in specifically an
>>> Integer.
>>>
>>> Michael
>>
>>
------------------------------
Message: 3
Date: Tue, 12 Jul 2011 19:16:06 +0000
From: Roelof Wobben <[email protected]>
Subject: [Haskell-beginners] safetail problem
To: <[email protected]>
Message-ID: <[email protected]>
Content-Type: text/plain; charset="iso-8859-1"
Hello,
As a exercise I need to rewrite the tail function into a safetail function by
using conditions.
So I made this : http://codepad.org/bKcCUdqy
But as you can see there is a error message.
Can anyone explain me how to solve this ?
Roelof
------------------------------
Message: 4
Date: Tue, 12 Jul 2011 15:21:46 -0400
From: Tom Murphy <[email protected]>
Subject: Re: [Haskell-beginners] safetail problem
To: Roelof Wobben <[email protected]>
Cc: [email protected]
Message-ID:
<CAO9Q0tW-xjQn6KaHs5BiAVh0mpsL0xM=a+bosbmjijwpu_d...@mail.gmail.com>
Content-Type: text/plain; charset=ISO-8859-1
_ isn't a value: it's a wildcard character for pattern matching.
Also, if x = [1,2,3], [0,x] is the wrong way to make a list. The right
way is (0:x) (":" is "cons")
What resource are you learning from?
On 7/12/11, Roelof Wobben <[email protected]> wrote:
>
>
> Hello,
>
>
>
> As a exercise I need to rewrite the tail function into a safetail function
> by using conditions.
>
>
>
> So I made this : http://codepad.org/bKcCUdqy
>
>
>
> But as you can see there is a error message.
>
>
>
> Can anyone explain me how to solve this ?
>
>
>
> Roelof
>
>
>
>
>
>
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://www.haskell.org/mailman/listinfo/beginners
>
------------------------------
Message: 5
Date: Tue, 12 Jul 2011 20:45:08 +0000
From: Roelof Wobben <[email protected]>
Subject: Re: [Haskell-beginners] safetail problem
To: <[email protected]>
Message-ID: <[email protected]>
Content-Type: text/plain; charset="iso-8859-1"
Hello,
I use Programming in Haskell from Graham Hutton.
But I can't find anything in the first 4 chapters from that book why I must use
(0,x) en what that means.
Roelof
----------------------------------------
> Date: Tue, 12 Jul 2011 15:21:46 -0400
> Subject: Re: [Haskell-beginners] safetail problem
> From: [email protected]
> To: [email protected]
> CC: [email protected]
>
> _ isn't a value: it's a wildcard character for pattern matching.
>
> Also, if x = [1,2,3], [0,x] is the wrong way to make a list. The right
> way is (0:x) (":" is "cons")
>
> What resource are you learning from?
>
> On 7/12/11, Roelof Wobben <[email protected]> wrote:
> >
> >
> > Hello,
> >
> >
> >
> > As a exercise I need to rewrite the tail function into a safetail function
> > by using conditions.
> >
> >
> >
> > So I made this : http://codepad.org/bKcCUdqy
> >
> >
> >
> > But as you can see there is a error message.
> >
> >
> >
> > Can anyone explain me how to solve this ?
> >
> >
> >
> > Roelof
> >
> >
> >
> >
> >
> >
> > _______________________________________________
> > Beginners mailing list
> > [email protected]
> > http://www.haskell.org/mailman/listinfo/beginners
> >
------------------------------
Message: 6
Date: Tue, 12 Jul 2011 16:53:31 -0400
From: Tom Murphy <[email protected]>
Subject: Re: [Haskell-beginners] safetail problem
To: Roelof Wobben <[email protected]>
Cc: [email protected]
Message-ID:
<CAO9Q0tUuohpfHkTdB+anHYk7u4nJBjDgFZQrEN=afjdxxd8...@mail.gmail.com>
Content-Type: text/plain; charset=ISO-8859-1
If:
x = [1,2,3] :: [Int]
y = [0] :: [Int]
z = 0
then:
[y,x] == [[0],[1,2,3]] :: [[Int]]
- a list of lists. It doesn't get simplified into [0,1,2,3].
(z,x) == (0,[1,2,3]) :: (Int, [Int])
- a tuple, whose first element is an Int, and whose second element
is a list of Ints ([Int])
z:x == [0,1,2,3] :: [Int]
The third one is what you're looking for. It's called "cons."
Tom
On 7/12/11, Roelof Wobben <[email protected]> wrote:
>
>
> Hello,
>
>
>
> I use Programming in Haskell from Graham Hutton.
>
>
>
> But I can't find anything in the first 4 chapters from that book why I must
> use (0,x) en what that means.
>
>
>
> Roelof
>
>
>
>
>
>
> ----------------------------------------
>> Date: Tue, 12 Jul 2011 15:21:46 -0400
>> Subject: Re: [Haskell-beginners] safetail problem
>> From: [email protected]
>> To: [email protected]
>> CC: [email protected]
>>
>> _ isn't a value: it's a wildcard character for pattern matching.
>>
>> Also, if x = [1,2,3], [0,x] is the wrong way to make a list. The right
>> way is (0:x) (":" is "cons")
>>
>> What resource are you learning from?
>>
>> On 7/12/11, Roelof Wobben <[email protected]> wrote:
>> >
>> >
>> > Hello,
>> >
>> >
>> >
>> > As a exercise I need to rewrite the tail function into a safetail
>> > function
>> > by using conditions.
>> >
>> >
>> >
>> > So I made this : http://codepad.org/bKcCUdqy
>> >
>> >
>> >
>> > But as you can see there is a error message.
>> >
>> >
>> >
>> > Can anyone explain me how to solve this ?
>> >
>> >
>> >
>> > Roelof
>> >
>> >
>> >
>> >
>> >
>> >
>> > _______________________________________________
>> > Beginners mailing list
>> > [email protected]
>> > http://www.haskell.org/mailman/listinfo/beginners
>> >
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://www.haskell.org/mailman/listinfo/beginners
>
------------------------------
Message: 7
Date: Tue, 12 Jul 2011 23:27:42 +0200
From: Martin Drautzburg <[email protected]>
Subject: [Haskell-beginners] Monads in javascript
To: beginners <[email protected]>
Message-ID: <[email protected]>
Content-Type: Text/Plain; charset="us-ascii"
Hello all,
I found some implementations of Monads in javascript, but they all do not
allow capturing the intermediate results. My question is: "what is the
ultimate cause for this"?
In haskell the second argument to (>>=) is a function a->Mb which can be
written as a lambda expression, e.g.
\x->[x].
In javascript such a function would be written as
function(x) {return[x]}.
Did I get this right: in haskell the chain of >>= is constructed from the end?
So chaining a->Mb and b->Mc gives you a->Mc, which is again suitable as a
second argument to (>>=), right?
So why can't I do this is javascipt? Or can I?
The reason I am asking this is because I am trying to beautify callbacks. An
asynchronous ajax call needs a function argument, which will be executed once
the call completes. But what if I want to take the results of the first call,
do something with them and pass it to a second ajax call, where I would again
have to pass another function argument. I would end up with a deeply nested
structure of lambdas, something like
f(a,b,...function(...){
...
g(... function(...)
I had some hopes that chaining functions monad-style would ease my pain. I
might be on the wrong track though, feel free to tell me so.
--
Martin
--
Martin
------------------------------
Message: 8
Date: Tue, 12 Jul 2011 17:48:37 -0400
From: David Place <[email protected]>
Subject: Re: [Haskell-beginners] safetail problem
To: Roelof Wobben <[email protected]>
Cc: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset=us-ascii
On Jul 12, 2011, at 4:45 PM, Roelof Wobben wrote:
> But I can't find anything in the first 4 chapters from that book why I must
> use (0,x) en what that means.
I think lists can be confusing for someone who is trying to learn Haskell and
has only used imperative languages before. Typically, those languages use
arrays to manipulate lists of things.
The meaning of lists in Haskell is hidden by the confusing but convenient
syntactic sugar used to write them. Really, lists are a recursive data type
with two cases.
The basis case: the empty list (written as "[]", close square bracket then
open square bracket)
The inductive case: a : List (here ":" is the constructor)
So, if we see a list written as [1,2,3,4] it really means (1 : (2 : (3 : (4 :
[]))))
All the programs that work on lists use the underlying representation, so you
really need to understand it.
This confusion goes all the way back to 1960. Blame it on John McCarthy,
inventor of Lisp!
I hope that is a little helpful.
Cheers,
David
____________________
David Place
Owner, Panpipes Ho! LLC
http://panpipesho.com
[email protected]
------------------------------
Message: 9
Date: Wed, 13 Jul 2011 00:03:44 +0200
From: Ertugrul Soeylemez <[email protected]>
Subject: Re: [Haskell-beginners] Monads in javascript
To: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset=US-ASCII
Martin Drautzburg <[email protected]> wrote:
> I found some implementations of Monads in javascript, but they all do
> not allow capturing the intermediate results. My question is: "what is
> the ultimate cause for this"?
>
> In haskell the second argument to (>>=) is a function a->Mb which can
> be written as a lambda expression, e.g.
> \x->[x].
> In javascript such a function would be written as
> function(x) {return[x]}.
>
> Did I get this right: in haskell the chain of >>= is constructed from
> the end?
Composition by (>>=) is by definition associative, so it really doesn't
matter how you set your parentheses.
> So chaining a->Mb and b->Mc gives you a->Mc, which is again suitable
> as a second argument to (>>=), right?
Yes, but beware that you may be confusing (>>=) with (>=>).
> So why can't I do this is javascipt? Or can I?
I don't see why that should be a problem. Note that there are many
different ways to represent monads. Haskell represents them in terms of
'fmap', 'return' and (>>=).
> The reason I am asking this is because I am trying to beautify
> callbacks. An asynchronous ajax call needs a function argument, which
> will be executed once the call completes. But what if I want to take
> the results of the first call, do something with them and pass it to a
> second ajax call, where I would again have to pass another function
> argument. I would end up with a deeply nested structure of lambdas,
> something like
>
> f(a,b,...function(...){
> ...
> g(... function(...)
>
> I had some hopes that chaining functions monad-style would ease my
> pain. I might be on the wrong track though, feel free to tell me so.
You probably are, because the usefulness of monads depends a good deal
on syntax. ECMA-based languages like JavaScript, ActionScript, Haxe,
etc. have a terrible syntax for anonymous functions.
To give you an alternative, this really sounds like you're looking for a
continuation-based solution. Continuation passing style could really
solve your problem elegantly even in JavaScript.
Greets,
Ertugrul
--
nightmare = unsafePerformIO (getWrongWife >>= sex)
http://ertes.de/
------------------------------
Message: 10
Date: Wed, 13 Jul 2011 00:25:15 +0200
From: Ertugrul Soeylemez <[email protected]>
Subject: Re: [Haskell-beginners] safetail problem
To: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset=US-ASCII
Roelof Wobben <[email protected]> wrote:
> As a exercise I need to rewrite the tail function into a safetail
> function by using conditions.
>
> So I made this : http://codepad.org/bKcCUdqy
>
> But as you can see there is a error message.
>
> Can anyone explain me how to solve this ?
Haskell is all about types. To understand Haskell, you really must
understand how types work. The square brackets are a syntactic
construct to easy constructing lists. As David pointed out, the
underlying representation is different. If the types don't make sense,
the whole program makes no sense. Let's see:
[1, [2, 3, 4]]
This cannot be right, because lists in Haskell are homogenous, so all
elements have the same type, but here you have a list of two elements,
one being 1 (a number) and the other being [2, 3, 4] (a list of
numbers). The types obviously don't fit, so the program cannot be
compiled. You have this:
x = 1
y = [2, 3, 4]
To put the element x in front of the list of elements y, you need a
function of the following type:
a -> [a] -> [a]
The (:) function has exactly that type, so you can write:
(:) x y
or to use the usually more convenient infix syntax:
x : y
This is about list construction. What you need here however is the
other direction: destruction. This is done by pattern matching. Again
as David pointed out a list is exactly a chain of (:) applications. The
list y would have the following real representation:
y = 2 : (3 : (4 : []))
Since (:) is right-associative you can leave out the parentheses to have
a slightly more readable equation. You can decompose this list by
pattern matching, which gives you access to the building blocks of the
list:
safeTail (x:xs) = xs
The (:) function is the "cons" or "prepended to" function. So safeTail
applied to x prepended to xs is equal to xs. One more equation and your
safeTail function is done.
One further note: I have intentionally left out almost all of the type
signatures here, but I strongly advise you to write them. In fact write
type signatures before writing the actual functions.
Greets,
Ertugrul
--
nightmare = unsafePerformIO (getWrongWife >>= sex)
http://ertes.de/
------------------------------
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