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Today's Topics:
1. Why WinGHCi does not complain with no import? (Angus Comber)
2. Re: Why WinGHCi does not complain with no import? (fa-ml)
3. Re: How to run Parser in chapter 8 Graham Hutton Haskell
book (Angus Comber)
4. Re: Why WinGHCi does not complain with no import?
(Daniel Trstenjak)
5. Picking apart getLine (Angus Comber)
6. Re: Picking apart getLine (David McBride)
----------------------------------------------------------------------
Message: 1
Date: Tue, 7 Jan 2014 13:58:37 +0000
From: Angus Comber <[email protected]>
To: Haskell Beginners <[email protected]>
Subject: [Haskell-beginners] Why WinGHCi does not complain with no
import?
Message-ID:
<caatguhwpccy_v7z9xtruvjfwub9c4uvbse-ostx3tygy7sq...@mail.gmail.com>
Content-Type: text/plain; charset="iso-8859-1"
I created my own version of getLine like this:
getLine' :: IO String
getLine' = do x <- getChar
if x == '\n' then
return []
else
do
xs <- getLine'
return (x:xs)
Basically I created a new file called test.hs and copied above code into
it. I opened WinGHCi afresh and :load test.hs
I can run getLine' in WinGHCi with no problem. But how does WinGHCi know
about getChar and IO?
I assumed I would have to import like this:
import System.IO
Is it because WinGHCi does this import for me?
If I write this program and invoke using runghci:
getLine' :: IO String
getLine' = do x <- getChar
if x == '\n' then
return []
else
do
xs <- getLine'
return (x:xs)
main = do
getLine'
It also works without complaining.
Is there a list of built-in libraries? Is that platform dependent?
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Message: 2
Date: Tue, 7 Jan 2014 15:05:44 +0100
From: fa-ml <[email protected]>
To: [email protected]
Subject: Re: [Haskell-beginners] Why WinGHCi does not complain with no
import?
Message-ID: <20140107140544.GA349@efikamx>
Content-Type: text/plain; charset=us-ascii
On Tue, Jan 07, 2014 at 01:58:37PM +0000, Angus Comber wrote:
> I can run getLine' in WinGHCi with no problem. But how does WinGHCi know
> about getChar and IO?
getChar is defined in the Prelude [1] (and the Prelude gets imported by default
in every program/module)
[1]
http://www.haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:getChar
------------------------------
Message: 3
Date: Tue, 7 Jan 2014 14:09:08 +0000
From: Angus Comber <[email protected]>
To: Haskell Beginners <[email protected]>
Subject: Re: [Haskell-beginners] How to run Parser in chapter 8 Graham
Hutton Haskell book
Message-ID:
<CAAtGUhXHovkgZyUL_q2+9gSCwYwSEm2C=7ejab2kxi6egms...@mail.gmail.com>
Content-Type: text/plain; charset="iso-8859-1"
For anyone else benefit, this is how I fixed issue.
1. download Parsing.lhs from author's website.
2. Also download parser.lhs and paste contents into my prog_haskell.hs
file, removing text comments and > symbols.
3. At top of my prog_haskell.hs file you need import Parsing
You can then call the functions in WinGHCi, eg eval "3+5*2"
For some reason I was not able to work out how to import both Parsing.lhs
and parser.lhs together. Will have to read up on modules later.
On 7 January 2014 11:43, Christian Maeder <[email protected]> wrote:
> Hi Angus,
>
> did you get an answer for your question? (Just pressing "reply" only
> answers to the list, as happened to my earlier answer to your initial
> question.)
>
> I don't know if WinGHCi works like ghci under linux (and I don't know
> which files Parsing.lhs and parser.lhs you tried to load.)
>
> At the ghci Prompt you can type ":browse" to see which names are in scope.
> Maybe parser.lhs could not be loaded for some reason that I cannot
> reproduce. (Maybe eval is explicitly not exported?)
>
> Cheers Christian
>
>
> Am 03.01.2014 18:43, schrieb Angus Comber:
>
>> I found some notes on the authors website explaining that the book code
>> would no longer work. So I downloaded two files: Parsing.lhs and
>> parser.lhs. In WinGHCi I then issued :load on both files.
>>
>> parser.lhs contains a function, eval, as follows:
>>
>> <other stuff above>
>> > eval :: String -> Int
>> > eval xs = case (parse expr xs) of
>> > [(n,[])] -> n
>> > [(_,out)] -> error ("unused input
>> " ++ out)
>> > [] -> error "invalid input"
>>
>> But in WinGHCi if I run: eval "2*3+4" I get error := Not in scope: `eval'
>>
>> I assume this is some sort of namespaces feature of Haskell that I have
>> not read about? Or I somehow have to state what library modules to
>> use??? Can anyone help?
>>
>>
>>
>>
>>
>>
>>
>> On 3 January 2014 15:58, Angus Comber <[email protected]
>> <mailto:[email protected]>> wrote:
>>
>> I am reading Chapter 8 of Programming Haskell by Graham Hutton and
>> trying to run the code in the book.
>>
>> It seems that there are already defined library implementations of
>> Parser so I used Parser' and generally ' on the end of each
>> identifier to attempt to eliminate this problem.
>>
>> So the code I copied from the book is:
>>
>> type Parser' a = String -> [(a, String)]
>>
>> return' :: a -> Parser' a
>> return' v = \x -> [(v, x)]
>>
>> failure' :: Parser' a
>> failure' = \inp -> []
>>
>> -- item doesn't seem to conflict with anything so no '
>> item :: Parser' Char
>> item = \inp -> case inp of
>> [] -> []
>> (x:xs) -> [(x,xs)]
>>
>>
>> parse' :: Parser' a -> String -> [(a, String)]
>> parse' p inp = p inp
>>
>>
>> p :: Parser' (Char, Char)
>> p = do x <- item
>> item
>> y <- item
>> return' (x,y)
>>
>> When run from WinGHCi I get error:
>>
>> prog_haskell.hs:458:9:
>> Couldn't match type `[(Char, String)]' with `Char'
>> Expected type: String -> [((Char, Char), String)]
>> Actual type: Parser' ([(Char, String)], [(Char, String)])
>> In the return type of a call of return'
>> In a stmt of a 'do' block: return' (x, y)
>> In the expression:
>> do { x <- item;
>> item;
>> y <- item;
>> return' (x, y) }
>>
>> 458.9 is line at end with return' (x,y)
>>
>>
>> Also in the chapter was the definition of >>= sequencing. as in:
>>
>> (>>=) :: Parser a -> (a -> Parser b) -> Parser b
>> p >>= f = \inp -> case parse p inp of
>> [] -> []
>> [(v,out)] -> parse (f v) out
>>
>> But I assumed this would already be in standard Haskell library and
>> so I didn't need to define. But even when I did add, still get same
>> error.
>>
>> How do I fix this?
>>
>>
>>
>>
>> _______________________________________________
>> Beginners mailing list
>> [email protected]
>> http://www.haskell.org/mailman/listinfo/beginners
>>
>>
>
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Message: 4
Date: Tue, 7 Jan 2014 15:36:17 +0100
From: Daniel Trstenjak <[email protected]>
To: [email protected]
Subject: Re: [Haskell-beginners] Why WinGHCi does not complain with no
import?
Message-ID: <20140107143617.GA15314@machine>
Content-Type: text/plain; charset=us-ascii
Hi Angus,
On Tue, Jan 07, 2014 at 01:58:37PM +0000, Angus Comber wrote:
> Is it because WinGHCi does this import for me?
It's defined by the Haskell standard, that you're automatically getting the
'Prelude' module:
http://haddocks.fpcomplete.com/fp/7.4.2/20130829-168/base/Prelude.html
Greetings,
Daniel
------------------------------
Message: 5
Date: Tue, 7 Jan 2014 16:54:15 +0000
From: Angus Comber <[email protected]>
To: Haskell Beginners <[email protected]>
Subject: [Haskell-beginners] Picking apart getLine
Message-ID:
<CAAtGUhWuGrEs60H4PbFRm_xO=s1nacz_phwtbcwt-31fhzv...@mail.gmail.com>
Content-Type: text/plain; charset="iso-8859-1"
Before looking at getLine, I can understand this:
getnumber :: IO Int
getnumber = do x <- getChar
if isDigit x then
return (ord x - ord '0')
else
return 0
OK, it is not a very useful function but at least I understand it. return
is required so that the function returns an IO Int.
But I don't understand this:
getLine' :: IO String
getLine' = do x <- getChar
if x == '\n' then
return []
else
do
xs <- getLine'
return (x:xs)
I can understand what will happen if a user enters a newline (only).
return [] brings the empty list into the monadic world.
But what is happening if x is not a newline?
xs <- getLine' will recursively call getChar and retrieve another character
from the input stream. But it will do this BEFORE the return (x:xs) - so
what is happening to all the head elements in the list - the x element?
It is difficult to picture in my mind how this is working. I understand
recursion but this looks tricky.
Can someone help me work this out?
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Message: 6
Date: Tue, 7 Jan 2014 12:15:02 -0500
From: David McBride <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Picking apart getLine
Message-ID:
<can+tr42k1_cb4_uefdif2ono2nzmuo_twqoswwpvbu8ftkz...@mail.gmail.com>
Content-Type: text/plain; charset=ISO-8859-1
You have the idea. The x is fetched with getChar, then it sits in
that context until the return is executed.
So the x is sitting there and getLine' is called. It makes its own x
via getChar, then maybe getLine' is called again. Each getLine' sits
there with its own version of x until finally the last getLine' get's
a \n, and then returns a []. Then the whole thing unwinds by
prepending x to [], then x to [x], then another x to [x,x], until
there are no more x's to return, and you have the whole string.
Hopefully that paragraph makes sense.
On Tue, Jan 7, 2014 at 11:54 AM, Angus Comber <[email protected]> wrote:
> Before looking at getLine, I can understand this:
>
> getnumber :: IO Int
> getnumber = do x <- getChar
> if isDigit x then
> return (ord x - ord '0')
> else
> return 0
>
> OK, it is not a very useful function but at least I understand it. return
> is required so that the function returns an IO Int.
>
> But I don't understand this:
>
> getLine' :: IO String
> getLine' = do x <- getChar
> if x == '\n' then
> return []
> else
> do
> xs <- getLine'
> return (x:xs)
>
>
> I can understand what will happen if a user enters a newline (only). return
> [] brings the empty list into the monadic world.
>
> But what is happening if x is not a newline?
>
> xs <- getLine' will recursively call getChar and retrieve another character
> from the input stream. But it will do this BEFORE the return (x:xs) - so
> what is happening to all the head elements in the list - the x element?
>
> It is difficult to picture in my mind how this is working. I understand
> recursion but this looks tricky.
>
> Can someone help me work this out?
>
>
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://www.haskell.org/mailman/listinfo/beginners
>
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