- Haskell-Cafe - Haskell.org

lazy strings and parallel read
by Manlio Perillo 19 Sep '08

19 Sep '08

Hi. After having read http://book.realworldhaskell.org/read/concurrent-and-multicore-programming.… I have a doubt about Data.ByteString.Lazy. Why getContents function don't use pread (or an emulation, if not available)? Thanks Manlio Perillo

3 6

A round of golf
by Creighton Hogg 19 Sep '08

19 Sep '08

Hey Haskell, So for a fairly inane reason, I ended up taking a couple of minutes and writing a program that would spit out, to the console, the number of lines in a file. Off the top of my head, I came up with this which worked fine with files that had 100k lines: main = do path <- liftM head $ getArgs h <- openFile path ReadMode n <- execStateT (countLines h) 0 print n untilM :: Monad m => (a -> m Bool) -> (a -> m ()) -> a -> m () untilM cond action val = do truthy <- cond val if truthy then return () else action val >> (untilM cond action val) countLines :: Handle -> StateT Int IO () countLines = untilM (\h -> lift $ hIsEOF h) (\h -> do lift $ hGetLine h modify (+1)) If this makes anyone cringe or cry "you're doing it wrong", I'd actually like to hear it. I never really share my projects, so I don't know how idiosyncratic my style is.

10 17

An interesting curiosity
by Andrew Coppin 19 Sep '08

19 Sep '08

I'm currently reading a C++ tutorial. (Don't ask!) Of course, C++ is that crazy language where *assignment* is actually an *operator*. Sick, sick people... The tutorial pointed out that "x = 4" is a legal assignment, but "4 = x" is not. "Obviously that would be silly." Now it occurs to me... in Haskell, "4 = x" is actually a perfectly legal thing to say. (!) It's pretty pointless, but it's legal. For example: foo x = let 4 = x in 2*x which, as best as I can tell, does exactly the same thing as (2*). In a similar vein, one might write foo x = do let 4 = x return (2*x) which also does more or less the same as (return). However, consider the following: foo :: Int -> Maybe Int foo x = do 4 <- return x return (2*x) This, it turns out, is an extremely odd way of checking whether x == 4. All of this works of course because in Haskell, "=" is not an assignment, it's a definition, and the RHS is not a variable, it's a pattern. And "4" is a perfectly legitimate pattern. Now, if only I could find a use for all this that borders on "useful"...! ;-)

8 7

Re: Strongly Typed Memory Areas
by oleg＠okmij.org 19 Sep '08

19 Sep '08

John Van Enk wrote: > Was Iavor/Mark's paper ever implemented as a GHC extension? > "Strongly Typed Memory Areas" It turns out most of the functionality is already available in Haskell: Lightweight static resources, for safe embedded and systems programming http://okmij.org/ftp/Haskell/types.html#ls-resources Chung-chieh Shan's TFP2007 talk specifically contrasted code from Iavor/Mark's paper with Haskell code. Strongly typed memory areas are possible in Haskell today.

1 0

Specifying a function type in a where clause.
by Rob deFriesse 18 Sep '08

18 Sep '08

I would like to know why I'm getting a particular compile time error message. In this program, I am specifying a function type on /combs'/ in the where clause: -------- module Main where import List (delete) combs :: Eq a => [a] -> Int -> [[a]] combs l 1 = map (\x -> [x]) l combs l n = foldl combs' [] l where combs' :: Eq a => [[a]] -> a -> [[a]] combs' acc x = let sl = delete x l in (map (\i -> x:i) $ combs sl (n-1)) ++ acc main = do print $ combs ["a","b","c","d"] 3 -------- I get this error message from GHC 6.8.3: cafe1.hs:9:43: Couldn't match expected type `a1' against inferred type `a' `a1' is a rigid type variable bound by the type signature for `combs'' at cafe1.hs:8:23 `a' is a rigid type variable bound by the type signature for `combs' at cafe1.hs:5:12 Expected type: [a1] Inferred type: [a] In the second argument of `delete', namely `l' In the expression: delete x l I don't understand why I'm seeing this. The type /a/ is the same type in /combs/ and /combs'/. I realize I can remove the type specification from /combs'/, and the code will compile. However, I'd like to get a better understanding of why GHC objects to this. Thank you, Rob.

3 3

Re: HStringTemplate to generate html tables
by Sterling Clover 18 Sep '08

18 Sep '08

Hmm... an untested suggestion... <table> $cells:{ row | <tr>$row:{ cell | <td>$cell$</td>}$</tr>}$ </table> (i.e. nested iteration) or to encompass the table call inside, $cells:{ row | <tr>$row:{ cell | <td>$cell$</td>}$</tr>}:{<table>$it$</table>}$ Regards, Sterl. On Thu, Sep 18, 2008 at 8:16 AM, Thomas Hartman < thomashartman1(a)googlemail.com> wrote: > I am trying to use HStringTemplate for generating html tables, for use > in http://happstutorial.com:5001 > > The best I could do is below. Seems awfully kludgy. > > Can someone suggest a better way, which keeps me inside the > StringTemplate way of doing things? (Eg, no suggestions to use HTML.* > libraries, which are nice for some things but are a higher barrier to > entry than just jumping in and writing html.) > > Some kind of map structure... for each... ? > > Thanks for any help! > > thomas. > > {-# LANGUAGE NoMonomorphismRestriction #-} > import Text.StringTemplate > import Control.Applicative > import qualified Text.PrettyPrint as PP > > main = putStrLn . PP.render . toPPDoc $ tableFromCells cells > cells = [["mee","mah","moh"],["fee","fah","foh"]] > > -- This does generate an html table, but seems horribly obfuscated. > -- is there machinery for doing things like html table generation > -- in some better way than this hack where </end><start> > -- tags are used as a separator? > -- I would imagine something like > -- newSTMP "$cells:{<td>$it$</td>}:{<tr>$it</tr>}:{<table>$it</table>}" > -- which works a bit like a list comprehension > tableFromCells cells = setAttribute "cells" cells . optInsertTmpl > [("separator","</td><td>")] > $ newSTMP "$cells:\ > \{<tr><td>$it$</td></tr>}:\ > \{<table>$it$</table>}$"

1 0

HStringTemplate to generate html tables
by Thomas Hartman 18 Sep '08

18 Sep '08

I am trying to use HStringTemplate for generating html tables, for use in http://happstutorial.com:5001 The best I could do is below. Seems awfully kludgy. Can someone suggest a better way, which keeps me inside the StringTemplate way of doing things? (Eg, no suggestions to use HTML.* libraries, which are nice for some things but are a higher barrier to entry than just jumping in and writing html.) Some kind of map structure... for each... ? Thanks for any help! thomas. {-# LANGUAGE NoMonomorphismRestriction #-} import Text.StringTemplate import Control.Applicative import qualified Text.PrettyPrint as PP main = putStrLn . PP.render . toPPDoc $ tableFromCells cells cells = [["mee","mah","moh"],["fee","fah","foh"]] -- This does generate an html table, but seems horribly obfuscated. -- is there machinery for doing things like html table generation -- in some better way than this hack where </end><start> -- tags are used as a separator? -- I would imagine something like -- newSTMP "$cells:{<td>$it$</td>}:{<tr>$it</tr>}:{<table>$it</table>}" -- which works a bit like a list comprehension tableFromCells cells = setAttribute "cells" cells . optInsertTmpl [("separator","</td><td>")] $ newSTMP "$cells:\ \{<tr><td>$it$</td></tr>}:\ \{<table>$it$</table>}$"

1 0

Real World HAppS: Cabalized, Self-Demoing HAppS Tutorial (Version 3)
by Thomas Hartman 18 Sep '08

18 Sep '08

I pushed a new version of happs-tutorial to the online demo at http://happstutorial.com:5001 This is also on hackage: cabal install happs-tutorial. (now version 3.) or darcs get http://code.haskell.org/happs-tutorial for the latest The demo/tutorial has the same basic functionality as the last release -- just a login form essentially -- but a lot more bling now. Like menu link items that change colors when the page is clicked. Also the login form that gives sane error messages. The focus for this release was on explaining how I used StringTemplate with HAppS. Hopefully in version 4 I'll finally get to State and Macid! And hopefully some functionality that actually does something beyond just showing what users have created logins :) Thomas

3 4

Re: [Haskell-cafe] Re: Float instance of 'read'
by Richard A. O'Keefe 18 Sep '08

18 Sep '08

On 18 Sep 2008, at 3:20 am, Mauricio wrote: > Agree about the answer, not about the question. The > correct one would be "is it possible to change haskell > syntax to support the international notation (not any > locally sensitive one) for decimal real numbers? Would > a change in 'read' be a good first step?" For some sense of "possible", the answer is clearly yes. However, it is perhaps misleading to call commas "THE international notation". The plain fact of the matter is that whatever any standards body may say, there are MANY notations for numbers. You might as well ask "is it possible to change Haskell syntax to support only the *real* Arabic digits ... for ... numbers". The Arabic script is used in many countries, so it's international. I don't read Arabic letters at all, but to me numbers written in Arabic script are no weirder than numbers using commas instead of dots. I would draw readers' attention to http://engineers.ihs.com/news/2006/nist-decimal-international.htm which says that (1) China, India, and Japan use decimal points, not commas. (2) There was a resolution of the CGPM in 2003 "endorsing the use of the point on the line as a decimal sign". (3) In June 2006, ISO agreed to allow the use of dots as decimal points in international standards. As it happens, the decimal point character I greatly prefer is the traditional British raised dot, but I can live with a low dot. I am 100% behind internationalised input and output, but allowing commas in numbers inside programming languages that already use commas for other purposes is, let's be polite, not a terribly good idea. You could probably get away with it in Lisp, but how many numbers are there in (1,2,3)? One thing that definitely is missing from Haskell numbers, and which I greatly miss, is a "thousands separator". Ada got around the dot/comma/apostrophe/whatever issue for thousands separators by using the underscore. For an unsigned decimal integer, this means allowing [0-9](_?[0-9])* instead of [0-9]+. It works really well, despite not being ANYBODY's cultural convention. > > > I know this looks difficult, but I'm sure it deserves at > least some thinking. We have previous examples for > less important issues: ghc does accept Windows end > of line conventions, the minus and sign needs special > syntax, and '.' can be used as decimal separator even > if it's use as function notation. > > Best, > Maurício > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > > > > ==================================================================== > Auto Generated Mail > Footer If this email is requesting your > password then it is a HOAX. > DO NOT reply to the email. > Validate this footer at the > Otago University web site keyword search: information security phish > ==================================================================== > > > -- If stupidity were a crime, who'd 'scape hanging?

3 2

Strongly Typed Memory Areas
by John Van Enk 18 Sep '08

18 Sep '08

Was Iavor/Mark's paper ever implemented as a GHC extension? "Strongly Typed Memory Areas" http://galois.com/down/2006_ID_Strongly.pdf -- /jve

1 0