Send Beginners mailing list submissions to
[email protected]
To subscribe or unsubscribe via the World Wide Web, visit
http://www.haskell.org/mailman/listinfo/beginners
or, via email, send a message with subject or body 'help' to
[email protected]
You can reach the person managing the list at
[email protected]
When replying, please edit your Subject line so it is more specific
than "Re: Contents of Beginners digest..."
Today's Topics:
1. Re: hlint and DoIfThenElse (Felipe Almeida Lessa)
2. Re: hlint and DoIfThenElse (Daniel Fischer)
3. Re: Beginners Digest, Vol 41, Issue 30 (Lee Short)
4. List comprehension question (Lee Short)
5. Re: List comprehension question (Daniel Fischer)
6. Re: Beginners Digest, Vol 41, Issue 30 (Daniel Fischer)
7. Precedence of Infix Operators in Do Syntax (Avery Robinson)
8. Re: Precedence of Infix Operators in Do Syntax (Brent Yorgey)
9. Re: Precedence of Infix Operators in Do Syntax
(Alexander Bernauer)
10. Re: List comprehension question (Alexander Bernauer)
----------------------------------------------------------------------
Message: 1
Date: Tue, 22 Nov 2011 21:04:36 -0200
From: Felipe Almeida Lessa <[email protected]>
Subject: Re: [Haskell-beginners] hlint and DoIfThenElse
To: Mike Meyer <[email protected]>
Cc: [email protected]
Message-ID:
<CANd=oggfcqz0enjrausys3hvq9r_kacx_o+2tjqhgzrttam...@mail.gmail.com>
Content-Type: text/plain; charset=UTF-8
On Tue, Nov 22, 2011 at 8:58 PM, Mike Meyer <[email protected]> wrote:
> You've just pressed one of my language-independent style hot
> buttons. Why on earth are you using an if/then/else here? What's wrong
> with the straightforward:
>
> ? ? ?return "" == results
>
> The expression results in a boolean, and it's even the one you want to
> return. So why not return it?
Minor nitpick since we are on 'begginers' list: you need parenthesis
there, so either
return ("" == results)
or
return $ "" == results
otherwise it would get parsed as '(return "") == results'.
Also, note that instead of doing "" ==, you could use 'null', so
return (null results)
HTH,
--
Felipe.
------------------------------
Message: 2
Date: Wed, 23 Nov 2011 00:06:04 +0100
From: Daniel Fischer <[email protected]>
Subject: Re: [Haskell-beginners] hlint and DoIfThenElse
To: [email protected]
Cc: Lee Short <[email protected]>
Message-ID: <[email protected]>
Content-Type: Text/Plain; charset="iso-8859-1"
On Tuesday 22 November 2011, 23:46:37, Lee Short wrote:
> hlint gives me a parse error on a clause using DoIfThenElse, even if I
> have the language pragma.
hlint probably has its own parser which doesn't yet implement that
extension.
> I don't see any hlint options to get around
> that, are there any?
Just indent the then and else lines a bit more.
>
> Is it considered good style to write code like this?
>
> if "" == results
> then return True
> else return False
>
> The obvious way rewrite below just seems clunky to me (though I can see
> how others might prefer it to the code above).
>
> return $ if "" == results
> then True
> else False
The obvious rewrite is
return (null results)
`if condition then True else False' is the same as `condition', which is
clearer (and shorter).
------------------------------
Message: 3
Date: Tue, 22 Nov 2011 15:21:33 -0800
From: Lee Short <[email protected]>
Subject: Re: [Haskell-beginners] Beginners Digest, Vol 41, Issue 30
To: <[email protected]>
Message-ID: <[email protected]>
Content-Type: text/plain; charset=UTF-8; format=flowed
> On Tue, 22 Nov 2011 14:46:37 -0800
> Lee Short <[email protected]> wrote:
>
>> hlint gives me a parse error on a clause using DoIfThenElse, even
>> if
>> I have the language pragma. I don't see any hlint options to get
>> around that, are there any?
>>
>> Is it considered good style to write code like this?
>>
>> if "" == results
>> then return True
>> else return False
>>
>> The obvious way rewrite below just seems clunky to me (though I can
>> see how others might prefer it to the code above).
>>
>> return $ if "" == results
>> then True
>> else False
>
> You've just pressed one of my language-independent style hot
> buttons. Why on earth are you using an if/then/else here? What's
> wrong
> with the straightforward:
>
> return "" == results
>
> The expression results in a boolean, and it's even the one you want
> to
> return. So why not return it?
>
That's not my actual code, I was just using it as a simplified example
and didn't put a lot of thought into it. My actual code is
if delete /= ""
then if exists
then deleteFromTable req t key
else return ()
else if exists
then updateTable req t key
else insertTable req t
I'd love to hear comments on how to make that more idiomatic, if
there's a way.
------------------------------
Message: 4
Date: Tue, 22 Nov 2011 16:08:26 -0800
From: Lee Short <[email protected]>
Subject: [Haskell-beginners] List comprehension question
To: <[email protected]>
Message-ID: <[email protected]>
Content-Type: text/plain; charset=UTF-8; format=flowed
So hlint suggests that I convert the line
checked = if ival==input then [("checked","checked")] else []
to
checked = [("checked","checked") | ival==input]
I'm not really sure why this works. Is there some implicit list being
filtered by ival==input? What's going on?
thanks
Lee
------------------------------
Message: 5
Date: Wed, 23 Nov 2011 01:22:55 +0100
From: Daniel Fischer <[email protected]>
Subject: Re: [Haskell-beginners] List comprehension question
To: [email protected]
Cc: Lee Short <[email protected]>
Message-ID: <[email protected]>
Content-Type: Text/Plain; charset="iso-8859-1"
On Wednesday 23 November 2011, 01:08:26, Lee Short wrote:
> So hlint suggests that I convert the line
>
> checked = if ival==input then [("checked","checked")] else []
>
> to
>
> checked = [("checked","checked") | ival==input]
>
> I'm not really sure why this works. Is there some implicit list being
> filtered by ival==input? What's going on?
A list comprehension
[expression | condition]
desugars to
if condition then [expression] else []
IMO, that's one of the places where hlint doesn't suggest an improvement.
------------------------------
Message: 6
Date: Wed, 23 Nov 2011 01:35:00 +0100
From: Daniel Fischer <[email protected]>
Subject: Re: [Haskell-beginners] Beginners Digest, Vol 41, Issue 30
To: [email protected]
Cc: Lee Short <[email protected]>
Message-ID: <[email protected]>
Content-Type: Text/Plain; charset="iso-8859-1"
On Wednesday 23 November 2011, 00:21:33, Lee Short wrote:
> That's not my actual code, I was just using it as a simplified example
Glad to read that.
> and didn't put a lot of thought into it. My actual code is
>
> if delete /= ""
> then if exists
> then deleteFromTable req t key
> else return ()
> else if exists
> then updateTable req t key
> else insertTable req t
>
> I'd love to hear comments on how to make that more idiomatic, if
> there's a way.
Hmm, harder, much harder.
You can replace the
if exists
then deleteFromTable req t key
else return ()
with
when exists (deleteFromTable req t key)
and instead of comparing to "", you can use null, so
if null delete
then if exists
then updateTable req t key
else insertTable req t
else when exists (deleteFromTable req t key)
I would recommend the when. Concerning the null vs. /= "", I prefer null,
but I'm not religious about it.
------------------------------
Message: 7
Date: Tue, 22 Nov 2011 23:03:51 -0500
From: Avery Robinson <[email protected]>
Subject: [Haskell-beginners] Precedence of Infix Operators in Do
Syntax
To: [email protected]
Message-ID:
<ca+6vedlku5tkwggytj3koy9lrwmhoxvecpn_-z3i305ijqr...@mail.gmail.com>
Content-Type: text/plain; charset="iso-8859-1"
Hello,
I was reading this block of code from
http://jasani.org/2008/02/18/command-line-haskell-and-error-handling-examples/
main = do
m <- hGetContents stdin
nums <- mapM readM . lines $ m
print (sum nums)
`catch` (\e -> hPutStrLn stderr ("couldn't sum lines: " ++ show e))
readM :: (Monad m, Read a) => String -> m a
readM s | [x] <- parse = return x
| otherwise = fail $ "Failed to parse \"" ++ s ++ "\" as a
number."
where
parse = [x | (x,_) <- reads s]
I don't understand how line 5 works. I thought that the do notation there
would be the same as:
main = hGetContents stdin >>= \m ->
mapM readM . lines $ m >>= \nums ->
print (sum nums) >>
`catch` (\e -> hPutStrLn stderr ("couldn't sum lines: " ++ show e))
But I don't understand how that makes any sense with that infix `catch`. It
looks like catch doesn't even have a first argument. Please enlighten me.
Thanks.
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://www.haskell.org/pipermail/beginners/attachments/20111122/b5ee87d7/attachment-0001.htm>
------------------------------
Message: 8
Date: Wed, 23 Nov 2011 01:51:43 -0500
From: Brent Yorgey <[email protected]>
Subject: Re: [Haskell-beginners] Precedence of Infix Operators in Do
Syntax
To: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset=us-ascii
On Tue, Nov 22, 2011 at 11:03:51PM -0500, Avery Robinson wrote:
> Hello,
>
> I was reading this block of code from
> http://jasani.org/2008/02/18/command-line-haskell-and-error-handling-examples/
>
> main = do
> m <- hGetContents stdin
> nums <- mapM readM . lines $ m
> print (sum nums)
> `catch` (\e -> hPutStrLn stderr ("couldn't sum lines: " ++ show e))
>
> readM :: (Monad m, Read a) => String -> m a
> readM s | [x] <- parse = return x
> | otherwise = fail $ "Failed to parse \"" ++ s ++ "\" as a
> number."
> where
> parse = [x | (x,_) <- reads s]
>
> I don't understand how line 5 works. I thought that the do notation there
> would be the same as:
>
> main = hGetContents stdin >>= \m ->
> mapM readM . lines $ m >>= \nums ->
> print (sum nums) >>
> `catch` (\e -> hPutStrLn stderr ("couldn't sum lines: " ++
> show e))
Some experimentation shows that in fact, the first argument of `catch` is the
*entire* do-block. That is, it is the same as
main = (do
m <- hGetContents stdin
nums <- mapM readM . lines $ m
print (sum nums)
)
`catch` (\e -> hPutStrLn stderr ("couldn't sum lines: " ++ show e))
However, I don't understand why. I would have thought it invalid.
Perversely, indenting the `catch` by one more space results in a valid
program with different behavior (since it only applies to the print).
-Brent
------------------------------
Message: 9
Date: Wed, 23 Nov 2011 11:15:59 +0100
From: Alexander Bernauer <[email protected]>
Subject: Re: [Haskell-beginners] Precedence of Infix Operators in Do
Syntax
To: [email protected]
Message-ID: <20111123101559.GC7634@apus>
Content-Type: text/plain; charset="us-ascii"
Hi
On Tue, Nov 22, 2011 at 11:03:51PM -0500, Avery Robinson wrote:
> main = do
> m <- hGetContents stdin
> nums <- mapM readM . lines $ m
> print (sum nums)
> `catch` (\e -> hPutStrLn stderr ("couldn't sum lines: " ++ show e))
Using Hammar's naive do-notation desugarer [1] your example code
becomes:
---8<---
main
= hGetContents stdin >>=
\ m -> mapM readM . lines $ m >>= \ nums -> print (sum nums)
`catch`
(\ e -> hPutStrLn stderr ("couldn't sum lines: " ++ show e))
--->8---
Comparing this with "ghc -ddump-ds" (cleaned up by removing _xyz
suffixes and type annotations)
---8<---
Main.main =
System.IO.Error.catch
(>>=
(GHC.IO.Handle.Text.hGetContents GHC.IO.Handle.FD.stdin)
(\ m ->
>>=
(GHC.Base.$
(GHC.Base..
(mapM readM)
Data.List.lines)
m)
(\ nums ->
print_aAV (sum_aAU nums))))
(\ e ->
System.IO.hPutStrLn
GHC.IO.Handle.FD.stderr
(GHC.Base.++
(GHC.Base.unpackCString# "couldn't sum lines: ")
(show e)))
--->8---
confirms this. But why is that?
The section on do expressions of the Haskell report [2] explains that in
the end a do block is just an expression.
And section 3.4 [3] states that "e1 op e2 = (op) e1 e2", which in our
case translates to "e1 `catch` e2 = catch e1 e2" where e1 is the
preceeding do block and e2 is the lambda expression.
So, the line starting with `catch` is not part of the do block any more.
Greetings
Alex
[1] https://gist.github.com/1341505
[2] http://www.haskell.org/onlinereport/exps.html#sect3.14
[3] http://www.haskell.org/onlinereport/exps.html#sect3.4
-------------- next part --------------
A non-text attachment was scrubbed...
Name: not available
Type: application/pgp-signature
Size: 198 bytes
Desc: Digital signature
URL:
<http://www.haskell.org/pipermail/beginners/attachments/20111123/986a8a20/attachment-0001.pgp>
------------------------------
Message: 10
Date: Wed, 23 Nov 2011 11:41:12 +0100
From: Alexander Bernauer <[email protected]>
Subject: Re: [Haskell-beginners] List comprehension question
To: Lee Short <[email protected]>
Cc: [email protected]
Message-ID: <20111123104111.GD7634@apus>
Content-Type: text/plain; charset="us-ascii"
Hi
Daniel already gave you an answer. The details on how list comprehension
is desugared can be found in section 3.11 of the Haskell report [1].
I am just mentioning this because I really want to encourage people to
read the language standard when hitting such a question.
Not saying one shouldn't post questions here. But it typically saves
_you_ a lot of time to just read the answer up. And if the standard is
well written - which it IMO is for Haskell - you even get a complete,
concise and unambigous answer which extands your knowledge of the
language.
Greetings
Alex
[1] http://www.haskell.org/onlinereport/exps.html#sect3.11
-------------- next part --------------
A non-text attachment was scrubbed...
Name: not available
Type: application/pgp-signature
Size: 198 bytes
Desc: Digital signature
URL:
<http://www.haskell.org/pipermail/beginners/attachments/20111123/0db34c0c/attachment.pgp>
------------------------------
_______________________________________________
Beginners mailing list
[email protected]
http://www.haskell.org/mailman/listinfo/beginners
End of Beginners Digest, Vol 41, Issue 31
*****************************************
