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Today's Topics:
1. A very counterintuitive behaviour of Haskell (Renzo Orsini)
2. Re: A very counterintuitive behaviour of Haskell
(Magnus Therning)
3. Re: A very counterintuitive behaviour of Haskell (Daniel Fischer)
4. Re: A very counterintuitive behaviour of Haskell (Antoine Latter)
5. Re: A very counterintuitive behaviour of Haskell (jean verdier)
6. Re: A very counterintuitive behaviour of Haskell
(Magnus Therning)
7. Re: A very counterintuitive behaviour of Haskell (Renzo Orsini)
8. Re: A very counterintuitive behaviour of Haskell
(Jonas Almstr?m Dureg?rd)
9. Re: A very counterintuitive behaviour of Haskell
(Brandon S Allbery KF8NH)
----------------------------------------------------------------------
Message: 1
Date: Thu, 27 Jan 2011 15:55:58 +0100
From: Renzo Orsini <[email protected]>
Subject: [Haskell-beginners] A very counterintuitive behaviour of
Haskell
To: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset=us-ascii
In studying Haskell, I produced the following output from GHC:
xxx-3:~ xxx$ GHCi
GHCi, version 6.12.3: http://www.haskell.org/ghc/ :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer-gmp ... linking ... done.
Loading package base ... linking ... done.
Loading package ffi-1.0 ... linking ... done.
Prelude> let f 7 = "ok"
Prelude> let f x = "no"
Prelude> f 3
"no"
Prelude> f 7
"no"
I suppose it is correct. However, for someone who is interested in the
language, it seems very counterintuitive... Somebody would be so kind to
explain to a neophyte this "feature" of the language?
Thank you very much.
Renzo
------------------------------
Message: 2
Date: Thu, 27 Jan 2011 15:02:22 +0000
From: Magnus Therning <[email protected]>
Subject: Re: [Haskell-beginners] A very counterintuitive behaviour of
Haskell
To: Renzo Orsini <[email protected]>
Cc: [email protected]
Message-ID:
<[email protected]>
Content-Type: text/plain; charset=UTF-8
On Thu, Jan 27, 2011 at 14:55, Renzo Orsini <[email protected]> wrote:
> In studying Haskell, I produced the following output from GHC:
>
> xxx-3:~ xxx$ GHCi
> GHCi, version 6.12.3: http://www.haskell.org/ghc/ ?:? for help
> Loading package ghc-prim ... linking ... done.
> Loading package integer-gmp ... linking ... done.
> Loading package base ... linking ... done.
> Loading package ffi-1.0 ... linking ... done.
> Prelude> let f 7 = "ok"
> Prelude> let f x = "no"
> Prelude> f 3
> "no"
> Prelude> f 7
> "no"
>
> I suppose it is correct. However, for someone who is interested in the
> language, it seems very counterintuitive... Somebody would be so kind to
> explain to a neophyte this "feature" of the language?
I suppose it comes down to pattern matching being on form, not on value.
/M
--
Magnus Therning ? ? ? ? ? ? ? ? ? ? ?OpenPGP: 0xAB4DFBA4
email: [email protected] ? jabber: [email protected]
twitter: magthe ? ? ? ? ? ? ? http://therning.org/magnus
------------------------------
Message: 3
Date: Thu, 27 Jan 2011 16:07:21 +0100
From: Daniel Fischer <[email protected]>
Subject: Re: [Haskell-beginners] A very counterintuitive behaviour of
Haskell
To: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset="iso-8859-1"
On Thursday 27 January 2011 15:55:58, Renzo Orsini wrote:
> In studying Haskell, I produced the following output from GHC:
>
> xxx-3:~ xxx$ GHCi
> GHCi, version 6.12.3: http://www.haskell.org/ghc/ :? for help
> Loading package ghc-prim ... linking ... done.
> Loading package integer-gmp ... linking ... done.
> Loading package base ... linking ... done.
> Loading package ffi-1.0 ... linking ... done.
> Prelude> let f 7 = "ok"
> Prelude> let f x = "no"
> Prelude> f 3
> "no"
> Prelude> f 7
> "no"
>
>
> I suppose it is correct. However, for someone who is interested in the
> language, it seems very counterintuitive... Somebody would be so kind to
> explain to a neophyte this "feature" of the language?
It's not a feature of the language, it's a feature of the interpreter.
In ghci's own words (evoked by the -Wall flag):
Prelude> let f 7 = "ok"
<interactive>:1:5:
Warning: Pattern match(es) are non-exhaustive
In an equation for `f':
Patterns not matched: #x with #x `notElem` [7#]
Prelude> let f x = "no"
<interactive>:1:5:
Warning: This binding for `f' shadows the existing binding
bound at <interactive>:1:5
<interactive>:1:7: Warning: Defined but not used: `x'
You have defined two independent functions, the second overwriting the
first binding.
To get what you wanted, you have to put the branches on the same line,
separated by a semicolon:
Prelude> let f 7 = "ok"; f _x = "no"
(0.02 secs, 3348380 bytes)
Prelude> f 3
"no"
(0.01 secs, 1962728 bytes)
Prelude> f 7
"ok"
(I prefixed the x with an underscore in the second equation to preven an
"unused variable" warning, could also have used the wildcard _).
>
> Thank you very much.
>
> Renzo
Cheers,
Daniel
------------------------------
Message: 4
Date: Thu, 27 Jan 2011 09:08:28 -0600
From: Antoine Latter <[email protected]>
Subject: Re: [Haskell-beginners] A very counterintuitive behaviour of
Haskell
To: Renzo Orsini <[email protected]>
Cc: [email protected]
Message-ID:
<[email protected]>
Content-Type: text/plain; charset=UTF-8
On Thu, Jan 27, 2011 at 8:55 AM, Renzo Orsini <[email protected]> wrote:
> In studying Haskell, I produced the following output from GHC:
>
> xxx-3:~ xxx$ GHCi
> GHCi, version 6.12.3: http://www.haskell.org/ghc/ ?:? for help
> Loading package ghc-prim ... linking ... done.
> Loading package integer-gmp ... linking ... done.
> Loading package base ... linking ... done.
> Loading package ffi-1.0 ... linking ... done.
> Prelude> let f 7 = "ok"
> Prelude> let f x = "no"
> Prelude> f 3
> "no"
> Prelude> f 7
> "no"
>
>
> I suppose it is correct. However, for someone who is interested in the
> language, it seems very counterintuitive... Somebody would be so kind to
> explain to a neophyte this "feature" of the language?
Here GHCi believes you've defined two functions.
Since they both have the same name, the most recent one wins :-)
Does that make sense?
Antoine
>
> Thank you very much.
>
> Renzo
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://www.haskell.org/mailman/listinfo/beginners
>
------------------------------
Message: 5
Date: Thu, 27 Jan 2011 16:09:53 +0100
From: jean verdier <[email protected]>
Subject: Re: [Haskell-beginners] A very counterintuitive behaviour of
Haskell
To: Renzo Orsini <[email protected]>
Cc: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset="UTF-8"
You have defined 2 functions that are called f and only the last
definition is used (f x = "no").
The definition you want should be written
let f 7 = "ok"; f x = "no"
so the function is defined once.
The problem comes from using the interpreter and not from haskell.
On Thu, 2011-01-27 at 15:55 +0100, Renzo Orsini wrote:
> In studying Haskell, I produced the following output from GHC:
>
> xxx-3:~ xxx$ GHCi
> GHCi, version 6.12.3: http://www.haskell.org/ghc/ :? for help
> Loading package ghc-prim ... linking ... done.
> Loading package integer-gmp ... linking ... done.
> Loading package base ... linking ... done.
> Loading package ffi-1.0 ... linking ... done.
> Prelude> let f 7 = "ok"
> Prelude> let f x = "no"
> Prelude> f 3
> "no"
> Prelude> f 7
> "no"
>
>
> I suppose it is correct. However, for someone who is interested in the
> language, it seems very counterintuitive... Somebody would be so kind to
> explain to a neophyte this "feature" of the language?
>
> Thank you very much.
>
> Renzo
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://www.haskell.org/mailman/listinfo/beginners
------------------------------
Message: 6
Date: Thu, 27 Jan 2011 15:31:23 +0000
From: Magnus Therning <[email protected]>
Subject: Re: [Haskell-beginners] A very counterintuitive behaviour of
Haskell
To: Renzo Orsini <[email protected]>
Cc: [email protected]
Message-ID:
<[email protected]>
Content-Type: text/plain; charset=UTF-8
On Thu, Jan 27, 2011 at 15:02, Magnus Therning <[email protected]> wrote:
> On Thu, Jan 27, 2011 at 14:55, Renzo Orsini <[email protected]> wrote:
>> In studying Haskell, I produced the following output from GHC:
>>
>> xxx-3:~ xxx$ GHCi
>> GHCi, version 6.12.3: http://www.haskell.org/ghc/ ?:? for help
>> Loading package ghc-prim ... linking ... done.
>> Loading package integer-gmp ... linking ... done.
>> Loading package base ... linking ... done.
>> Loading package ffi-1.0 ... linking ... done.
>> Prelude> let f 7 = "ok"
>> Prelude> let f x = "no"
>> Prelude> f 3
>> "no"
>> Prelude> f 7
>> "no"
>>
>> I suppose it is correct. However, for someone who is interested in the
>> language, it seems very counterintuitive... Somebody would be so kind to
>> explain to a neophyte this "feature" of the language?
>
> I suppose it comes down to pattern matching being on form, not on value.
What an amazingly wrong answer by me, sorry for the noise ;-)
/M
--
Magnus Therning ? ? ? ? ? ? ? ? ? ? ?OpenPGP: 0xAB4DFBA4
email: [email protected] ? jabber: [email protected]
twitter: magthe ? ? ? ? ? ? ? http://therning.org/magnus
------------------------------
Message: 7
Date: Thu, 27 Jan 2011 16:31:36 +0100
From: Renzo Orsini <[email protected]>
Subject: Re: [Haskell-beginners] A very counterintuitive behaviour of
Haskell
To: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset=us-ascii
Thank you very much to everybody. Your explanation is perfectly clear! I was
reading the tutorial at http://learnyouahaskell.com/syntax-in-functions .
Initially the tutorial cited GHCi, so I mistakenly wrote the functions on
different lines.
Thank very much again. I regained the desire of continuing my study... :-)
Renzo
On Jan 27, 2011, at 16:09 , jean verdier wrote:
> You have defined 2 functions that are called f and only the last
> definition is used (f x = "no").
> The definition you want should be written
> let f 7 = "ok"; f x = "no"
> so the function is defined once.
> The problem comes from using the interpreter and not from haskell.
>
>
> On Thu, 2011-01-27 at 15:55 +0100, Renzo Orsini wrote:
>> In studying Haskell, I produced the following output from GHC:
>>
>> xxx-3:~ xxx$ GHCi
>> GHCi, version 6.12.3: http://www.haskell.org/ghc/ :? for help
>> Loading package ghc-prim ... linking ... done.
>> Loading package integer-gmp ... linking ... done.
>> Loading package base ... linking ... done.
>> Loading package ffi-1.0 ... linking ... done.
>> Prelude> let f 7 = "ok"
>> Prelude> let f x = "no"
>> Prelude> f 3
>> "no"
>> Prelude> f 7
>> "no"
>>
>>
>> I suppose it is correct. However, for someone who is interested in the
>> language, it seems very counterintuitive... Somebody would be so kind to
>> explain to a neophyte this "feature" of the language?
>>
>> Thank you very much.
>>
>> Renzo
>> _______________________________________________
>> Beginners mailing list
>> [email protected]
>> http://www.haskell.org/mailman/listinfo/beginners
>
>
------------------------------
Message: 8
Date: Thu, 27 Jan 2011 16:35:27 +0100
From: Jonas Almstr?m Dureg?rd <[email protected]>
Subject: Re: [Haskell-beginners] A very counterintuitive behaviour of
Haskell
To: jean verdier <[email protected]>
Cc: [email protected]
Message-ID:
<[email protected]>
Content-Type: text/plain; charset=ISO-8859-1
> The problem comes from using the interpreter and not from haskell.
But a very similar 'problem' can be achieved in generic Haskell:
x = do
let f 7 = "yes"
let f x = "no"
putStrLn (f 7)
Arguably the compiler could issue a warning here because the second
'let' keyword might be intended to be spaces. I don't know how common
these mistakes are though.
I think the original poster suggests that the shadowing function
should only be used when the original function doesn't match. That is
more problematic if you ask me though.
/J
On 27 January 2011 16:09, jean verdier <[email protected]> wrote:
> You have defined 2 functions that are called f and only the last
> definition is used (f x = "no").
> The definition you want should be written
> let f 7 = "ok"; f x = "no"
> so the function is defined once.
> The problem comes from using the interpreter and not from haskell.
>
>
> On Thu, 2011-01-27 at 15:55 +0100, Renzo Orsini wrote:
>> In studying Haskell, I produced the following output from GHC:
>>
>> xxx-3:~ xxx$ GHCi
>> GHCi, version 6.12.3: http://www.haskell.org/ghc/ ?:? for help
>> Loading package ghc-prim ... linking ... done.
>> Loading package integer-gmp ... linking ... done.
>> Loading package base ... linking ... done.
>> Loading package ffi-1.0 ... linking ... done.
>> Prelude> let f 7 = "ok"
>> Prelude> let f x = "no"
>> Prelude> f 3
>> "no"
>> Prelude> f 7
>> "no"
>>
>>
>> I suppose it is correct. However, for someone who is interested in the
>> language, it seems very counterintuitive... Somebody would be so kind to
>> explain to a neophyte this "feature" of the language?
>>
>> Thank you very much.
>>
>> Renzo
>> _______________________________________________
>> 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: 9
Date: Thu, 27 Jan 2011 12:09:27 -0500
From: Brandon S Allbery KF8NH <[email protected]>
Subject: Re: [Haskell-beginners] A very counterintuitive behaviour of
Haskell
To: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset=UTF-8
-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1
On 1/27/11 09:55 , Renzo Orsini wrote:
> Prelude> let f 7 = "ok"
> Prelude> let f x = "no"
Doing it in separate let bindings doesn't combine; instead, the second one
hides the first. let {f 7 = "ok"; f x = "no"} is more likely to do what you
expect.
- --
brandon s. allbery [linux,solaris,freebsd,perl] [email protected]
system administrator [openafs,heimdal,too many hats] [email protected]
electrical and computer engineering, carnegie mellon university KF8NH
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------------------------------
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