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Today's Topics:
1. Re: Pattern matching on binary trees ([email protected])
2. Re: Meaning of (Eq a) (Hein Hundal)
3. Re: Meaning of (Eq a) (Markus L?ll)
4. Trouble in installing Packages (Lorenzo Isella)
5. Re: Meaning of (Eq a) (Brandon S Allbery KF8NH)
----------------------------------------------------------------------
Message: 1
Date: Sun, 5 Sep 2010 15:37:03 +0530
From: "[email protected]" <[email protected]>
Subject: Re: [Haskell-beginners] Pattern matching on binary trees
To: Rohit Garg <[email protected]>
Cc: [email protected]
Message-ID:
<[email protected]>
Content-Type: text/plain; charset="utf-8"
Hi Rohit,
You can probably ignore the first line of my reply, if it feels a bit
confusing :)
On Sun, Sep 5, 2010 at 3:16 PM, [email protected] <
[email protected]> wrote:
> The idea behind pattern matching is that you can use any of the data
> constructor for the type to dismantle the value coming in.
>
> For depthTree, the type of value expected is Tree a. The Node constructor
> is used to dismantle the incoming value into three parts and bind it to
> variables x u v. Note here that depthTree accepts only one argument as input
> and the type of that value is (Tree a)
>
> When you do not want to dismantle the incoming value, then you would not
> use any data constructor. Your function would then become,
> depthTree :: Tree a -> Int
> depthTree x = ..
>
> You cannot use depthTree (x u v) because the compiler would then try to
> look for a data constructor x . And in Haskell, data constructor names begin
> with a uppercase letter.
>
> You can refer to this page on Haskell wiki
> bookhttp://en.wikibooks.org/wiki/Haskell/Pattern_matching for
> more information.
>
>
> On Sun, Sep 5, 2010 at 2:18 PM, Rohit Garg <[email protected]> wrote:
>
>> Hi,
>>
>> I am working on exercises in RWH, chapter 3 and I have a a question
>> about the pattern matching in the depth-of-tree question.
>>
>> The tree datatype is defined as
>>
>> data Tree a = Node a (Tree a) (Tree a)
>> | Empty
>> deriving (Show)
>>
>> And to pattern match on it, I need to write
>>
>> depthTree (Node x u v) = 1 + max (depthTree u) (depthTree v)
>>
>> My question is why do I need to prefix x with Node? Why can't it
>> pattern match on depthTree (x u v)? More generally, where do I need to
>> put data constructors in pattern matches?
>>
>> --
>> Rohit Garg
>>
>> http://rpg-314.blogspot.com/
>> _______________________________________________
>> Beginners mailing list
>> [email protected]
>> http://www.haskell.org/mailman/listinfo/beginners
>>
>
>
>
> --
> Regards
> Lakshmi Narasimhan T V
>
--
Regards
Lakshmi Narasimhan T V
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------------------------------
Message: 2
Date: Sun, 5 Sep 2010 03:20:46 -0700 (PDT)
From: Hein Hundal <[email protected]>
Subject: Re: [Haskell-beginners] Meaning of (Eq a)
To: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset=iso-8859-1
> Date: Sun, 5 Sep 2010 14:28:09 +0530
> From: Rohit Garg <[email protected]>
>
> RWH: chapter 3 - in question 5, you have to write a
> function which determines if a list is a palindrome.
> Here is my solution
>
> isPalindrome :: (Eq a) => [a] -> Bool
> isPalindrome [] = False
> isPalindrome x = compareLists x (reverse x)
> where compareLists [x] [y] = x == y
> compareLists (x:xs) (y:ys) = if x == y
> then compareLists xs ys
> else False
>
> Although it works, my question is why ghci refuses to
> run it without the "(Eq a) => " being added to the type
> signature of the function. Presumably, it is to let ghc
> know that you can perform equality tests on a. If so,
> then why does the sumList function below work without
> any type signature of any kind? I haven't told ghc
> that the input list elements can be added together.
>
> sumList [] = 0
> sumList (x:xs) = x + sumList xs
When I compile isPalindrome with ghc 6.12.1, I do not get any errors even if I
drop the type signature. If I drop the type signature and use ghci to get the
type signature (":t isPalindrome"), then I get
*Main> :t isPalindrome
isPalindrome :: (Eq t) => [t] -> Bool
So, it seems that ghc 6.12.1 correctly infers the types.
Cheers,
Hein
PS: Haskell does equality of lists, so you can also use the definition
isPalindrome v = v == reverse v
------------------------------
Message: 3
Date: Sun, 5 Sep 2010 14:12:24 +0300
From: Markus L?ll <[email protected]>
Subject: Re: [Haskell-beginners] Meaning of (Eq a)
To: Hein Hundal <[email protected]>, [email protected]
Message-ID:
<[email protected]>
Content-Type: text/plain; charset=ISO-8859-1
I have not tested, but I suspect, that if you don't provide a type
signature for either isPalindrome or sumList, it will get inferred
correctly. The problem is that if you *do* provide a typesignature for
either of them, you *have* to give it a properly constrained
typesignatures.
I think for sumList a signature of [a] -> a does not work, because you
use the adding function, which has type (Num a) -> a -> a -> a. So
when providing a signature for sumList you have to add the Num a
constraint too. The same goes for isPlindrome, where Eq a is
neccessary because of the type of ==.
Like, for isPalindrome the signature [a] -> Bool is not correct,
because it says any 'a' will work (because it is not constrained), but
any 'a' wont work -- it has to be an equitable 'a', thus the (Eq a)
constraint.
The point is, that when providing a signature, it has to be
sufficently (or more) constrained -- GHC won't patch it wor you, but
if the signature is missing it will (try to) infer it.
Markus
On Sun, Sep 5, 2010 at 1:20 PM, Hein Hundal <[email protected]> wrote:
>> Date: Sun, 5 Sep 2010 14:28:09 +0530
>> From: Rohit Garg <[email protected]>
>>
>> RWH: chapter 3 - in question 5, you have to write a
>> function which determines if a list is a palindrome.
>> Here is my solution
>>
>> isPalindrome :: (Eq a) => [a] -> Bool
>> isPalindrome [] = False
>> isPalindrome x = compareLists x (reverse x)
>> where compareLists [x] [y] = x == y
>> compareLists (x:xs) (y:ys) = if x == y
>> then compareLists xs ys
>> else False
>>
>> Although it works, my question is why ghci refuses to
>> run it without the "(Eq a) => " being added to the type
>> signature of the function. Presumably, it is to let ghc
>> know that you can perform equality tests on a. If so,
>> then why does the sumList function below work without
>> any type signature of any kind? I haven't told ghc
>> that the input list elements can be added together.
>>
>> sumList [] = 0
>> sumList (x:xs) = x + sumList xs
>
> When I compile isPalindrome with ghc 6.12.1, I do not get any errors even if
> I drop the type signature. If I drop the type signature and use ghci to get
> the type signature (":t isPalindrome"), then I get
>
>
> *Main> :t isPalindrome
> isPalindrome :: (Eq t) => [t] -> Bool
>
>
> So, it seems that ghc 6.12.1 correctly infers the types.
>
> Cheers,
> Hein
>
> PS: Haskell does equality of lists, so you can also use the definition
>
> isPalindrome v = v == reverse v
>
>
>
>
>
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://www.haskell.org/mailman/listinfo/beginners
>
------------------------------
Message: 4
Date: Sun, 05 Sep 2010 16:05:27 +0200
From: Lorenzo Isella <[email protected]>
Subject: [Haskell-beginners] Trouble in installing Packages
To: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Dear All,
I am quite new to Haskell and have only very recently found out about cabal.
I am interested in some haskell packages for scientific applications,
but I am experiencing some problems in installing them on my machine
(ubuntu 10.04, amd64 architecture).
For instance, I pasted below the output of my attempts to install
hmatrix-static.
Any idea about how to fix this?
Cheers
Lorenzo
$ cabal install hmatrix-static
Resolving dependencies...
Configuring array-0.2.0.0...
Preprocessing library array-0.2.0.0...
Building array-0.2.0.0...
[ 1 of 10] Compiling Data.Array ( Data/Array.hs,
dist/build/Data/Array.o )
Data/Array.hs:1:0:
Warning: Module `Prelude' is deprecated:
You are using the old package `base' version 3.x.
Future GHC versions will not support base version 3.x. You
should update your code to use the new base version 4.x.
Data/Array/IO.hs:1:15:
Warning: -#include is deprecated: No longer has any effect
Data/Array/IO/Internals.hs:1:15:
Warning: -#include is deprecated: No longer has any effect
[ 2 of 10] Compiling Data.Array.Base ( Data/Array/Base.hs,
dist/build/Data/Array/Base.o )
Data/Array/Base.hs:1:0:
Warning: Module `Prelude' is deprecated:
You are using the old package `base' version 3.x.
Future GHC versions will not support base version 3.x. You
should update your code to use the new base version 4.x.
Data/Array/Base.hs:1177:8:
Warning: Bindings containing unlifted types must use an outermost
bang pattern:
{ W# e# = if initialValue then maxBound else 0 }
*** This will be an error in GHC 6.14! Fix your code now!
In the definition of `newArray':
newArray (l, u) initialValue
= ST $ \ s1# -> case safeRangeSize (l, u) of {
(n@(I# n#)) -> ... }
where
W# e# = if initialValue then maxBound else 0
In the instance declaration for `MArray (STUArray s) Bool (ST s)'
Data/Array/Base.hs:1489:8:
Warning: Bindings containing unlifted types must use an outermost
bang pattern:
{ I# last# = 8 * 8 - 1 }
*** This will be an error in GHC 6.14! Fix your code now!
In the definition of `bOOL_SCALE':
bOOL_SCALE n#
= (n# +# last#) `uncheckedIShiftRA#` 3#
where
I# last# = 8 * 8 - 1
Data/Array/Base.hs:1491:8:
Warning: Bindings containing unlifted types must use an outermost
bang pattern:
{ I# last# = 8 * 8 - 1 }
*** This will be an error in GHC 6.14! Fix your code now!
In the definition of `bOOL_WORD_SCALE':
bOOL_WORD_SCALE n#
= bOOL_INDEX (n# +# last#)
where
I# last# = 8 * 8 - 1
Data/Array/Base.hs:1492:37:
Warning: Bindings containing unlifted types must use an outermost
bang pattern:
{ I# scale# = 8 }
*** This will be an error in GHC 6.14! Fix your code now!
In the definition of `wORD_SCALE':
wORD_SCALE n#
= scale# *# n#
where
I# scale# = 8
Data/Array/Base.hs:1493:37:
Warning: Bindings containing unlifted types must use an outermost
bang pattern:
{ I# scale# = 8 }
*** This will be an error in GHC 6.14! Fix your code now!
In the definition of `dOUBLE_SCALE':
dOUBLE_SCALE n#
= scale# *# n#
where
I# scale# = 8
Data/Array/Base.hs:1494:37:
Warning: Bindings containing unlifted types must use an outermost
bang pattern:
{ I# scale# = 4 }
*** This will be an error in GHC 6.14! Fix your code now!
In the definition of `fLOAT_SCALE':
fLOAT_SCALE n#
= scale# *# n#
where
I# scale# = 4
Data/Array/Base.hs:1505:8:
Warning: Bindings containing unlifted types must use an outermost
bang pattern:
{ W# mask# = 8 * 8 - 1 }
*** This will be an error in GHC 6.14! Fix your code now!
In the definition of `bOOL_BIT':
bOOL_BIT n#
= int2Word# 1#
`uncheckedShiftL#`
(word2Int# (int2Word# n# `and#` mask#))
where
W# mask# = 8 * 8 - 1
Data/Array/Base.hs:1506:47:
Warning: Bindings containing unlifted types must use an outermost
bang pattern:
{ W# mb# = maxBound }
*** This will be an error in GHC 6.14! Fix your code now!
In the definition of `bOOL_NOT_BIT':
bOOL_NOT_BIT n#
= bOOL_BIT n# `xor#` mb#
where
W# mb# = maxBound
[ 3 of 10] Compiling Data.Array.IArray ( Data/Array/IArray.hs,
dist/build/Data/Array/IArray.o )
Data/Array/IArray.hs:1:0:
Warning: Module `Prelude' is deprecated:
You are using the old package `base' version 3.x.
Future GHC versions will not support base version 3.x. You
should update your code to use the new base version 4.x.
[ 4 of 10] Compiling Data.Array.MArray ( Data/Array/MArray.hs,
dist/build/Data/Array/MArray.o )
Data/Array/MArray.hs:1:0:
Warning: Module `Prelude' is deprecated:
You are using the old package `base' version 3.x.
Future GHC versions will not support base version 3.x. You
should update your code to use the new base version 4.x.
[ 5 of 10] Compiling Data.Array.IO.Internals (
Data/Array/IO/Internals.hs, dist/build/Data/Array/IO/Internals.o )
Data/Array/IO/Internals.hs:1:0:
Warning: Module `Prelude' is deprecated:
You are using the old package `base' version 3.x.
Future GHC versions will not support base version 3.x. You
should update your code to use the new base version 4.x.
[ 6 of 10] Compiling Data.Array.IO ( Data/Array/IO.hs,
dist/build/Data/Array/IO.o )
Data/Array/IO.hs:73:13:
`haFD' is not a (visible) field of constructor `Handle__'
Data/Array/IO.hs:73:22:
`haBuffer' is not a (visible) field of constructor `Handle__'
Data/Array/IO.hs:73:36:
`haIsStream' is not a (visible) field of constructor `Handle__'
Data/Array/IO.hs:74:5: Not in scope: data constructor `Buffer'
Data/Array/IO.hs:74:13:
`bufBuf' is not a (visible) field of constructor `Buffer'
Data/Array/IO.hs:74:25:
`bufWPtr' is not a (visible) field of constructor `Buffer'
Data/Array/IO.hs:74:36:
`bufRPtr' is not a (visible) field of constructor `Buffer'
Data/Array/IO.hs:75:4: Not in scope: `bufferEmpty'
Data/Array/IO.hs:82:24:
`bufWPtr' is not a (visible) constructor field name
Data/Array/IO.hs:82:35:
`bufRPtr' is not a (visible) constructor field name
Data/Array/IO.hs:86:24:
`bufRPtr' is not a (visible) constructor field name
Data/Array/IO.hs:95:27:
Not in scope: type constructor or class `RawBuffer'
Data/Array/IO.hs:101:10: Not in scope: `readRawBuffer'
Data/Array/IO.hs:125:22:
`haFD' is not a (visible) field of constructor `Handle__'
Data/Array/IO.hs:125:31:
`haBuffer' is not a (visible) field of constructor `Handle__'
Data/Array/IO.hs:125:45:
`haIsStream' is not a (visible) field of constructor `Handle__'
Data/Array/IO.hs:127:18: Not in scope: data constructor `Buffer'
Data/Array/IO.hs:127:26:
`bufBuf' is not a (visible) field of constructor `Buffer'
Data/Array/IO.hs:127:42:
`bufWPtr' is not a (visible) field of constructor `Buffer'
Data/Array/IO.hs:127:53:
`bufSize' is not a (visible) field of constructor `Buffer'
Data/Array/IO.hs:135:30:
`bufWPtr' is not a (visible) constructor field name
Data/Array/IO.hs:142:7: Not in scope: data constructor `Buffer'
Data/Array/IO.hs:142:15:
`bufBuf' is not a (visible) field of constructor `Buffer'
Data/Array/IO.hs:142:27:
`bufState' is not a (visible) field of constructor `Buffer'
Data/Array/IO.hs:142:36:
Not in scope: data constructor `WriteBuffer'
Data/Array/IO.hs:143:8:
`bufRPtr' is not a (visible) field of constructor `Buffer'
Data/Array/IO.hs:143:19:
`bufWPtr' is not a (visible) field of constructor `Buffer'
Data/Array/IO.hs:143:34:
`bufSize' is not a (visible) field of constructor `Buffer'
Data/Array/IO.hs:151:22:
Not in scope: type constructor or class `RawBuffer'
Data/Array/IO.hs:151:43:
Not in scope: type constructor or class `RawBuffer'
Data/Array/IO.hs:153:22:
Not in scope: type constructor or class `RawBuffer'
Data/Array/IO.hs:153:35:
Not in scope: type constructor or class `RawBuffer'
cabal: Error: some packages failed to install:
array-0.2.0.0 failed during the building phase. The exception was:
ExitFailure 1
containers-0.3.0.0 depends on array-0.2.0.0 which failed to install.
cpphs-1.11 depends on array-0.2.0.0 which failed to install.
haskell-src-exts-1.2.0 depends on array-0.2.0.0 which failed to install.
haskell-src-meta-0.0.6 depends on array-0.2.0.0 which failed to install.
haskell98-1.0.1.1 depends on array-0.2.0.0 which failed to install.
hmatrix-0.7.2.1 depends on array-0.2.0.0 which failed to install.
hmatrix-static-0.3 depends on array-0.2.0.0 which failed to install.
packedstring-0.1.0.1 depends on array-0.2.0.0 which failed to install.
template-haskell-2.4.0.0 depends on array-0.2.0.0 which failed to install.
tfp-0.2 depends on array-0.2.0.0 which failed to install.
------------------------------
Message: 5
Date: Sun, 05 Sep 2010 10:47:40 -0400
From: Brandon S Allbery KF8NH <[email protected]>
Subject: Re: [Haskell-beginners] Meaning of (Eq a)
To: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset=UTF-8
-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1
On 9/5/10 04:58 , Rohit Garg wrote:
> Although it works, my question is why ghci refuses to run it without
> the "(Eq a) => " being added to the type signature of the function.
Haskell won't do type inference if you provide a function with a type
signature, so if you say anything at all you need to say everything about
it. If you say *nothing* about it, as with the second function, Haskell
will infer everything including required constraints.
- --
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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