July 2013 - Beginners

Some guidance
by Peter Hall 07 Jul '13

07 Jul '13

Hi, I'm working on a little library to trade bitcoins via the MtGox API. https://github.com/peterjoel/auto-trader I haven't got very far yet, but I feel like I'm already overcomplicating things, especially the module structure. The reason it has so many modules, even though there is hardly any code, is partly because I wanted to expose the properties of the MtGoxTicker record as lenses, without polluting the namespace with the underlying _accessors. Is this good or is there a more usual way of dealing with the problem? The other problem I'm facing is with name collisions for record accessors. There are two versions of MtGoxTicker from different services - one with a subset of the properties, which is supposed to be faster (it isn't, but that isn't the point). The data types are here https://github.com/peterjoel/auto-trader/blob/6974d66ae51459479c19be291d075…. One is commented one out while I decide what to do. What is the best way to model those records to avoid collisions, while not being confusing to users of the library? I am very tempted to use type classes, but that feels naughty. Using unique prefixes seems bad too - it would be nice for some code to be able to use them interchangeably if they don't need all the fields. Is it possible to use unique names for the actual record accessors, but share the names of their lenses? Thanks, Peter

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Memory usage prediction
by Emanuel Koczwara 04 Jul '13

04 Jul '13

Hi, Where can I find some informations abut memory usage prediction? In C (or C++) while I write code I can predict how much stack space will be used, how much heap space will be used, I can count instructions. I want to see all these things while I'm looking at my code. Is it possible in Haskell? Best regards, Emanuel

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High order function beggining doubt
by Igor Pinheiro Leão 03 Jul '13

03 Jul '13

Hi there, let a function whith the following signature: annexer :: (b -> c) -> [a -> b] -> [a -> c] How can i do that? Kind regards, Igor -- Igor Vinícius Graduando em Ciência da Computação

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Question about precedence
by Marc Gorenstein 02 Jul '13

02 Jul '13

What sort of beast is ( / 8) in Haskell? It looks like it is a function that divides a number by 8. *Main> ( / 8 ) 4 0.5 *Main> let a = ( / 8 ) *Main> a 4 0.5 -- Yup that works. Does ( / 8 ) turn into a function that takes an argument, so that the "left" input to the /, becomes a right input to the function? What precedence rule is being followed here? Thanks, Marc

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Still confused
by Marc Gorenstein 01 Jul '13

01 Jul '13

Hi Brandon, Darren, and Michael, Thanks for you responses, but I'm still confused. Here are two examples of operator sections. The first takes the infix operator / and turns it into a prefix operator. Prelude> let eight_div_by = ((/) 8 ) Prelude> eight_div_by 4 2.0 I get that. But look at the following: We now have a prefix operator with the input on the "wrong" side. Prelude> let div_by_eight = ( / 8 ) Prelude> div_by_eight 4 0.5 Why should ( / 8) 4 = 0.5? Thanks again, Marc

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Re: [Haskell-beginners] Parsing email headers (alex + happy vs parsec)
by Tim Holland 30 Jun '13

30 Jun '13

Hi Roger, On the contrary, I think that a parser combinator is a very modular solution. Each type of parseable string can be recursively defined with the monadic functions of the combinator. These functions, of course, don't care about context and can be used wherever appropriate. If you send me a test file of headers and related outputs, I might see if I can whip something up to try to change your mind. Regards, Tim Holland On 28 June 2013 09:40, <beginners-request(a)haskell.org> wrote: > Send Beginners mailing list submissions to > beginners(a)haskell.org > > 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 > beginners-request(a)haskell.org > > You can reach the person managing the list at > beginners-owner(a)haskell.org > > When replying, please edit your Subject line so it is more specific > than "Re: Contents of Beginners digest..." > > > Today's Topics: > > 1. Re: How to Lex, Parse, and Serialize-to-XML email messages > (Costello, Roger L.) > > > ---------------------------------------------------------------------- > > Message: 1 > Date: Fri, 28 Jun 2013 16:40:01 +0000 > From: "Costello, Roger L." <costello(a)mitre.org> > Subject: Re: [Haskell-beginners] How to Lex, Parse, and > Serialize-to-XML email messages > To: "The Haskell-Beginners Mailing List - Discussion of primarily > beginner-level topics related to Haskell" <beginners(a)haskell.org> > Message-ID: > <B5FEE00B53CF054AA8439027E8FE17751EFA91FE(a)IMCMBX04.MITRE.ORG> > Content-Type: text/plain; charset="iso-8859-1" > > Hi Tim, > > > ? I realize you've already finished with the project ... > > Actually, your message comes at an excellent time. I am not finished with > the project. I have only finished one of the email headers -- the From > Header. > > Just today I was wondering how to proceed next: > > > - Should I extend my parser so that it deals with each of the > other email headers? That is, create one monolithic parser for the entire > email message? That doesn't seem very modular. I don't think that Happy > supports importing other Happy parsers. Ideally I would create a parser for > the From Header, a parser for the To Header, a parser for the Subject > Header, and so forth. Then I would import each of them to create one > unified email parser. If Happy doesn't support importing, I figured it > might be better to switch to something that can combine parsers - a parser > combinator - such as parsec. Unfortunately, I don't know anything about > Parsec, but am eager to learn. > > - I wonder if I can use Happy to generate individual parsers - a > parser for the From Header, a parser for the To Header, a parser for the > Subject Header - and then use Parsec to combine them? > > As you see Tim, your suggestion to use parsec falls on receptive ears. I > welcome all suggestions. > > /Roger > > From: beginners-bounces(a)haskell.org [mailto:beginners-bounces@haskell.org] > On Behalf Of Tim Holland > Sent: Friday, June 28, 2013 12:18 PM > To: beginners(a)haskell.org > Subject: Re: [Haskell-beginners] How to Lex, Parse, and Serialize-to-XML > email messages > > Hi Roger, > > I realize you've already finished with the project, but for the future I > think its a lot easier to use a parser combinator with Text.Parsec and > Text.Parsec.String to do a similar thing. For example, if you were parsing > XML to get a parse a single tag, you would try something like this: > > parseTag :: Parser Tag > parseTag = many1 alphanum <?> "tag" > > To get a tagged form, try > parseTagged :: Parser (Tag, [Elem]) > parseTagged = do > char '<' > name <- parseTag > char '>' > content <- many (try parseElem) > string "</" > parseTag > char '>' > return (name, content) > <?> "tagged form" > > and so one. I haven't tried this out, but a parser similar to yours would > go something like this: > > --Datatypes > type DisplayName = String > type EmailAddress = String > data Mailbox = Mailbox DisplayName EmailAddress deriving (Show) > > parseFromHeader :: Parser [Mailbox] > parseFromHeader = do > string "From: " > mailboxes = many (try parseMailbox) > return mailboxes > > parseMailbox :: Parser Mailbox > parseMailbox = do > parseComments > -- Names are optional > parseComments > name <- try parseDisplayName > parseComments > address <- parseEmailAddress > parseComments > try char ',' > return Mailbox name address > <?> "Parse an indidivuals mailbox" > > parseEmailAddress :: Parser EmailAddress > parseEmailAddress = do > try char '<' > handle <- many1 (noneof "@") -- Or whatever is valid here > char '@' > domain <- parseDomain > try char '<' > return handle++@++domain > > parseDomain :: Parser String > parseDomain = > (char '[' >> parseDomain >>= (\domainName -> do char ']' > return domainName)) > <|> parseWebsiteName >>= return > > And so on. Again, I've tested none of the Email header bits but the XML > bit works. It requires some level of comfort with monadic operations, but > beyond that I think it's a much simpler may to parse. > > Regards, > Tim Holland > > > > > > On 28 June 2013 03:00, <beginners-request(a)haskell.org<mailto: > beginners-request(a)haskell.org>> wrote: > Send Beginners mailing list submissions to > beginners(a)haskell.org<mailto:beginners@haskell.org> > > 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 > beginners-request(a)haskell.org<mailto:beginners-request@haskell.org > > > > You can reach the person managing the list at > beginners-owner(a)haskell.org<mailto:beginners-owner@haskell.org> > > When replying, please edit your Subject line so it is more specific > than "Re: Contents of Beginners digest..." > > > Today's Topics: > > 1. data declaration using other type's names? (Patrick Redmond) > 2. Re: data declaration using other type's names? (Brandon Allbery) > 3. Re: data declaration using other type's names? (Nikita Danilenko) > 4. Re: what to do about excess memory usage (Chadda? Fouch?) > 5. Re: what to do about excess memory usage (James Jones) > 6. How to Lex, Parse, and Serialize-to-XML email messages > (Costello, Roger L.) > > > ---------------------------------------------------------------------- > > Message: 1 > Date: Thu, 27 Jun 2013 11:24:51 -0400 > From: Patrick Redmond <plredmond(a)gmail.com<mailto:plredmond@gmail.com>> > Subject: [Haskell-beginners] data declaration using other type's > names? > To: beginners(a)haskell.org<mailto:beginners@haskell.org> > Message-ID: > <CAHUea4FfBP8L1kU+tS1-2cVPvAB4h22j35JcNwRC-jGds0= > v6g(a)mail.gmail.com<mailto:v6g@mail.gmail.com>> > Content-Type: text/plain; charset=UTF-8 > > Hey Haskellers, > > I noticed that ghci lets me do this: > > > data Foo = Int Int | Float > > :t Int > Int :: Int -> Foo > > :t Float > Float :: Foo > > :t Int 4 > Int 4 :: Foo > > It's confusing to have type constructors that use names of existing > types. It's not intuitive that the name "Int" could refer to two > different things, which brings me to: > > > data Bar = Bar Int > > :t Bar > Bar :: Int -> Bar > > Yay? I can have a simple type with one constructor named the same as the > type. > > Why is this allowed? Is it useful somehow? > > --Patrick > > > > ------------------------------ > > Message: 2 > Date: Thu, 27 Jun 2013 11:37:46 -0400 > From: Brandon Allbery <allbery.b(a)gmail.com<mailto:allbery.b@gmail.com>> > Subject: Re: [Haskell-beginners] data declaration using other type's > names? > To: The Haskell-Beginners Mailing List - Discussion of primarily > beginner-level topics related to Haskell <beginners(a)haskell.org > <mailto:beginners@haskell.org>> > Message-ID: > < > CAKFCL4U-E4B_+cts0vpNX8Ar9wccQDjgzWOYHLXLsLAv+Qn_cg(a)mail.gmail.com<mailto: > CAKFCL4U-E4B_%2Bcts0vpNX8Ar9wccQDjgzWOYHLXLsLAv%2BQn_cg(a)mail.gmail.com>> > Content-Type: text/plain; charset="utf-8" > > On Thu, Jun 27, 2013 at 11:24 AM, Patrick Redmond <plredmond(a)gmail.com > <mailto:plredmond@gmail.com>>wrote: > > > I noticed that ghci lets me do this: > > > > Not just ghci, but ghc as well. > > > > Yay? I can have a simple type with one constructor named the same as the > > type. > > Why is this allowed? Is it useful somehow? > > > > It's convenient for pretty much the situation you showed, where the type > constructor and data constructor have the same name. A number of people do > advocate that it not be used, though, because it can be confusing for > people. (Not for the compiler; data and type constructors can't be used in > the same places, it never has trouble keeping straight which is which.) > > It might be best to consider this as "there is no good reason to *prevent* > it from happening, from a language standpoint". > > -- > brandon s allbery kf8nh sine nomine > associates > allbery.b(a)gmail.com<mailto:allbery.b@gmail.com> > ballbery(a)sinenomine.net<mailto:ballbery@sinenomine.net> > unix, openafs, kerberos, infrastructure, xmonad > http://sinenomine.net >

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