Recursion with do??
Hal Daume III
hdaume@ISI.EDU
Fri, 14 Mar 2003 10:19:04 -0800 (PST)
There are two types of 'let' in Haskell. There's let/in, which is an
expression, and let (from do), which is a statement.
This means that any place you would write an expression (for instance,
after the '=' sign in function definitions), you can write:
let <declarations> in <expr>
contrast this with do notation, where you a let is a statement (just like
"x <- foo" is a statement) and doesn't have a corresponding "in".
--
Hal Daume III | hdaume@isi.edu
"Arrest this man, he talks in maths." | www.isi.edu/~hdaume
On Fri, 14 Mar 2003, Alexandre Weffort Thenorio wrote:
> Thanks it works but what exactly does this "in" before the last statement
> does?? I have never seen it before and the program gives me the parse error
> if I don't have it??
>
> Best Regards
>
> NooK
>
> ----- Original Message -----
> From: "Hal Daume III" <hdaume@ISI.EDU>
> To: "Alexandre Weffort Thenorio" <alethenorio@home.se>
> Sent: Friday, March 14, 2003 4:32 PM
> Subject: Re: Recursion with do??
>
>
> > > fbk size new_str (x:xs) = do
> > > let first = findstr size new_str x
> > > let bksize = (Function that gives the size of the string to
> substitute in
> > > this line)
> > > let done = fbk bksize new_str xs
> > > return done
> >
> > There's no reason to use do notation here. This is a pure function. YOu
> > can write it as:
> >
> > > fbk size new_str (x:xs) =
> > > let first = ...
> > > bksize = ...
> > > in fbk bksize new_str xs
> >
> > I'm not sure why you would be getting a parse error though.
> >
> >
> > > ----- Original Message -----
> > > From: "Hal Daume III" <hdaume@ISI.EDU>
> > > To: "Alexandre Weffort Thenorio" <alethenorio@home.se>
> > > Sent: Thursday, March 13, 2003 9:33 PM
> > > Subject: Re: How to search for a string sequence in a file a rewrite
> it???
> > >
> > >
> > > > what is the type signature of write, and why are you using do
> > > > notation? might you want something like:
> > > >
> > > > fillIn lines = do
> > > > let bkfilled = findstr str str2 lines
> > > > write bkfilled
> > > >
> > > > or something like that?
> > > >
> > > > --
> > > > Hal Daume III | hdaume@isi.edu
> > > > "Arrest this man, he talks in maths." | www.isi.edu/~hdaume
> > > >
> > > > On Thu, 13 Mar 2003, Alexandre Weffort Thenorio wrote:
> > > >
> > > > > Ooops a small error before but here is the right one.
> > > > >
> > > > > Great. I got almost everything. My problem now is:
> > > > >
> > > > > I got a function called findstr where
> > > > >
> > > > > findstr "aaaa" "zzzzz" ["xxxxaaaa","xxxaaaaxxx"] =
> > > > > ["xxxxzzzzz","xxxzzzzzxxx"]
> > > > >
> > > > > and then I try something like
> > > > >
> > > > > fillIn lines = do
> > > > > bkfilled <- (findstr str str2 lines)
> > > > > write bkfilled
> > > > >
> > > > > where write takes a [String] and concatenates it writing it to a
> file
> > > then.
> > > > > But I get this error saying:
> > > > >
> > > > > Expected Type: [String]
> > > > > Inferred Type: String
> > > > > In the first argument of 'write' namelly 'bkfilled'
> > > > > In a do expection pattern binding: write bkfilled
> > > > >
> > > > > Any idea?? I mean bkfilled is supposed to be [String] but it says it
> is
> > > a
> > > > > String, any idea why???
> > > > >
> > > > > Best Regards
> > > > >
> > > > > Alex
> > > > >
> > > > > ----- Original Message -----
> > > > > From: "Hal Daume III" <hdaume@ISI.EDU>
> > > > > To: "Alexandre Weffort Thenorio" <alethenorio@home.se>
> > > > > Cc: <haskell@haskell.org>
> > > > > Sent: Thursday, March 13, 2003 1:38 AM
> > > > > Subject: Re: How to search for a string sequence in a file a rewrite
> > > it???
> > > > >
> > > > >
> > > > > > Right. *Recurse* down the list. Somethign like:
> > > > > >
> > > > > > foo orig_str new_str xl@(x:xs)
> > > > > > | orig_str `isPrefixOf` xl = <something>
> > > > > > | otherwise = x : <something>
> > > > > >
> > > > > > --
> > > > > > Hal Daume III | hdaume@isi.edu
> > > > > > "Arrest this man, he talks in maths." |
> www.isi.edu/~hdaume
> > > > > >
> > > > > > On Thu, 13 Mar 2003, Alexandre Weffort Thenorio wrote:
> > > > > >
> > > > > > > Well the problem is that the string I want to replace will
> usually
> > > be in
> > > > > the
> > > > > > > middle of a line in the text file and then is PrefixOf of no
> use??
> > > Any
> > > > > other
> > > > > > > suggestion??
> > > > > > >
> > > > > > > Like the text file will be similar to
> > > > > > >
> > > > > > > abcdedjkfhlafl
> > > > > > > sajkhlabbbbsf
> > > > > > > akfhjklafjkhfk
> > > > > > > sdfasfsaasffaa
> > > > > > >
> > > > > > > So I want to replace this bbbb in the middle there with aaaa. I
> > > could
> > > > > run
> > > > > > > lines on the file but from there i have no idea. I have got an
> idea
> > > but
> > > > > for
> > > > > > > it to work I need to know if there is a function that I can
> replace
> > > a
> > > > > > > certain Char in a string by another Char by knowing the index of
> > > this
> > > > > char??
> > > > > > >
> > > > > > > Best Regards
> > > > > > >
> > > > > > > NooK
> > > > > > > ----- Original Message -----
> > > > > > > From: "Hal Daume III" <hdaume@ISI.EDU>
> > > > > > > To: "Alexandre Weffort Thenorio" <alethenorio@home.se>
> > > > > > > Cc: <haskell@haskell.org>
> > > > > > > Sent: Thursday, March 13, 2003 1:05 AM
> > > > > > > Subject: Re: How to search for a string sequence in a file a
> rewrite
> > > > > it???
> > > > > > >
> > > > > > >
> > > > > > > > This is how I would do it:
> > > > > > > >
> > > > > > > > recurse down the input string. use isPrefixOf to check is the
> > > string
> > > > > youw
> > > > > > > > ant to replace is at the head of the string. if it is, 'drop'
> the
> > > > > > > > appropriate number of characters and stick the replacement
> string
> > > on
> > > > > the
> > > > > > > > front. then recurse.
> > > > > > > >
> > > > > > > > this should be about 3 lines of code.
> > > > > > > >
> > > > > > > > --
> > > > > > > > Hal Daume III |
> hdaume@isi.edu
> > > > > > > > "Arrest this man, he talks in maths." |
> > > www.isi.edu/~hdaume
> > > > > > > >
> > > > > > > > On Thu, 13 Mar 2003, Alexandre Weffort Thenorio wrote:
> > > > > > > >
> > > > > > > > > OK Guys. First I would like to say thanks for all the help
> given
> > > > > before.
> > > > > > > As
> > > > > > > > > I said I am still learning Haskell. My problem is the
> following:
> > > > > > > > >
> > > > > > > > > I have a text file and somewhere in the file there is string
> > > (Let's
> > > > > say
> > > > > > > > > aaaa). So I need to find this exact string and overwrite
> with
> > > > > another
> > > > > > > string
> > > > > > > > > (Lets say bbbb).
> > > > > > > > >
> > > > > > > > > I have thought a bit and couldn't find any solution, and I
> > > believe
> > > > > it is
> > > > > > > a
> > > > > > > > > piece of cake for some of you. Is there a method already
> > > implemented
> > > > > in
> > > > > > > > > haskell to do that??
> > > > > > > > > Anyway thanks in advance.
> > > > > > > > >
> > > > > > > > > Best Regards
> > > > > > > > >
> > > > > > > > > NooK
> > > > > > > > >
> > > > > > > > > ----- Original Message -----
> > > > > > > > > From: "Arthur Baars" <arthurb@cs.uu.nl>
> > > > > > > > > To: <haskell@haskell.org>
> > > > > > > > > Cc: "Norman Ramsey" <nr@eecs.harvard.edu>
> > > > > > > > > Sent: Wednesday, March 12, 2003 1:06 AM
> > > > > > > > > Subject: Re: clueless GHCI user wishes to load QuickCheck
> > > > > > > > >
> > > > > > > > >
> > > > > > > > > > QuickCheck is in de "util" package. You can load a package
> > > with
> > > > > the
> > > > > > > > > > -package flag:
> > > > > > > > > >
> > > > > > > > > > $ ghci -package util
> > > > > > > > > > Prelude> :browse QuickCheck
> > > > > > > > > > class Arbitrary a where {
> > > > > > > > > > arbitrary :: Gen a; coarbitrary :: forall b. a -> Gen
> > > b ->
> > > > > Gen
> > > > > > > b; }
> > > > > > > > > > arbitrary :: forall a. (Arbitrary a) => Gen a
> > > > > > > > > > ...
> > > > > > > > > > Prelude>:module QuickCheck
> > > > > > > > > >
> > > > > > > > > > Prelude QuickCheck> :info trivial
> > > > > > > > > > -- trivial is a variable
> > > > > > > > > > trivial :: forall a. (Testable a) => Bool -> a -> Property
> > > > > > > > > >
> > > > > > > > > > Hope this helps.
> > > > > > > > > >
> > > > > > > > > > Cheers,
> > > > > > > > > >
> > > > > > > > > > Arthur
> > > > > > > > > >
> > > > > > > > > > On Tuesday, March 11, 2003, at 07:02 PM, Norman Ramsey
> wrote:
> > > > > > > > > >
> > > > > > > > > > > Can anyone help me figure out how to load QuickCheck
> into
> > > GHCI?
> > > > > > > > > > > QuickCheck is included in my Debian package, but my
> attempts
> > > > > > > > > > > at loading it are bootless:
> > > > > > > > > > >
> > > > > > > > > > > Prelude> :load QuickCheck
> > > > > > > > > > > can't find module `QuickCheck'
> > > > > > > > > > > Prelude> :load util/QuickCheck
> > > > > > > > > > > can't find module `util/QuickCheck'
> > > > > > > > > > > Prelude> :info
> > > > > > > > > > > syntax: `:i <thing-you-want-info-about>'
> > > > > > > > > > > Prelude> :load util/QuickCheck.hi
> > > > > > > > > > > can't find module `util/QuickCheck.hi'
> > > > > > > > > > > Prelude> :load
> > > /usr/lib/ghc-5.02.2/imports/util/QuickCheck.hi
> > > > > > > > > > > can't find module
> > > > > `/usr/lib/ghc-5.02.2/imports/util/QuickCheck.hi'
> > > > > > > > > > > Prelude> :load
> /usr/lib/ghc-5.02.2/imports/util/QuickCheck
> > > > > > > > > > > can't find module
> > > `/usr/lib/ghc-5.02.2/imports/util/QuickCheck'
> > > > > > > > > > > Prelude>
> > > > > > > > > > >
> > > > > > > > > > > Any advice, anyone?
> > > > > > > > > > >
> > > > > > > > > > >
> > > > > > > > > > > Norman
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