- Haskell-Cafe - Haskell.org

ExitFailure 127
by Sebastian Schulz 16 Oct '01

16 Oct '01

hi. I'm trying to run an system command (wget) within a CGI script and get the following error: >Exitfailure 127 How can I get more information out of an ExitFailure x error? Can I use ioeGetErrorString? But this function doesn't work on ExitCode, which system returns. TIA sebastian

1 0

fixed point
by Song Li 15 Oct '01

15 Oct '01

Hi, anybody can explain how Y combinator work in find fixed point ? say : Y = 'lam' f . ('lam' x . f(xx))('lam' x . f(xx)) X = Y F -> ( 'lam' x .F(xx))('lam' x . F(xx)) -> F(('lam' x . F(xx))('lam' x . F(xx))) <- How can this expression be derived ???????????? ->F X <- And why ? Thanks a lot! Song

1 0

a newbie's question
by Song Li 14 Oct '01

14 Oct '01

Hi, there, Could anybody explain what does this type defination mean : data xxx a = xxx (a ->b) looks xxx can use itself as constructor(like tree) but change the type...... Thanks! Song

2 1

Closed Classes
by Eric Allen Wohlstadter 12 Oct '01

12 Oct '01

Are there any plans to add closed classes as a hugs extension? This was described in the paper: Object-Oriented Style Overloading for Haskell, Mark Shields and Simon Peyton Jones; BABEL workshop '01. Eric Wohlstadter

1 0

Re: = vs ->
by Ashley Yakeley 12 Oct '01

12 Oct '01

At 2001-10-09 11:55, Mark Carroll wrote: >What is the rationale for when Haskell demands a "=" and when it demands >a "->"? What? Example please... -- Ashley Yakeley, Seattle WA

7 10

Re: performUnsafeIO
by Ashley Yakeley 11 Oct '01

11 Oct '01

At 2001-10-11 11:55, Sebastian Schulz wrote: >I need to extract an IO String to String. No!!! Don't do it. Make the final result of your Haskell calculation be an "IO String". Then when you have a look at it, it will be automatically executed and you'll see the String. -- Ashley Yakeley, Seattle WA

1 0

help for exercise 4.10
by rock dwan 11 Oct '01

11 Oct '01

Iam having some difficulties doing exercise 4.10 from craft of functional programming book second edition ..is their a possible solution for this ? _________________________________________________________________ Get your FREE download of MSN Explorer at http://explorer.msn.com/intl.asp

2 1

performUnsafeIO
by Sebastian Schulz 11 Oct '01

11 Oct '01

hi. I need to extract an IO String to String. Remembering some function like 'performUnsafeIO :: IO a -> a', I searched the hugs libs, but without a match. Does a function like this still exists? Or how can I transform IO a to a? regards seb

3 2

Re: Reasons behind the "one instance per type" limitation
by Ashley Yakeley 10 Oct '01

10 Oct '01

At 2001-10-08 09:27, Diego Dainese wrote: >what are the reasons behind the rule stating that a type must not be >declared as an instance of a particular class, more than once in the >program? It's so that the members of the class are unambiguous. -- class C t where foo :: t -> Integer instance C Bool where foo _ = 3; instance C Bool where foo _ = 5; ambiguous = foo True; -- GHC has a flag that will turn the rule off. -- Ashley Yakeley, Seattle WA

4 4

Re: = vs ->
by Ashley Yakeley 10 Oct '01

10 Oct '01

At 2001-10-09 17:36, Mark Carroll wrote: >So, for instance, how come function definitions and guards use "=" but >lambdas and cases use "->"? It's like this: f x = fx f = \x -> fx f :: X -> FX f x = case x of x' -> fx' x'' -> fx'' Make sense? -- Ashley Yakeley, Seattle WA

1 0