April 2005 - Hugs-Users

WinHugs error
by Paqui Lucio 26 Feb '06

26 Feb '06

Hi, > I have installed Hugs98 Nov2003 on Windows XP. > WinHugs opens with the message: > > __ __ __ __ ____ ___ > _______________________________________________ > || || || || || || ||__ Hugs 98: Based on the Haskell 98 standard > ||___|| ||__|| ||__|| __|| Copyright (c) 1994-2003 > ||---|| ___|| World Wide Web: http://haskell.org/hugs > || || Report bugs to: hugs-bugs(a)haskell.org > || || Version: Nov 2003 > _______________________________________________ > > Hugs mode: Restart with command line option +98 for Haskell 98 mode > > ERROR ".\Prelude.hs" - Module "Hugs.Prelude" already loaded > Hugs.Prelude> > > > > Is that normal ? > > Thanks in advance, > Paqui > > --------------------------------- > Paqui Lucio > Dpto de LSI > Facultad de Informática > Paseo Manuel de Lardizabal, 1 > 20080-San Sebastián > SPAIN > --------------------------------- > Tfn: (+34) (9)43 015049 > Fax: (+34) (9)43 015590 > --------------------------------- >

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Re: [Haskell-cafe] Writing functions in Haskell.
by Daniel Fischer 05 May '05

05 May '05

Am Freitag, 29. April 2005 18:01 schrieb Ross Paterson: > On Thu, Apr 28, 2005 at 09:22:15PM -0400, Cale Gibbard wrote: > > The second problem arises from hugs not doing the same thing as ghci > > with respect to looking up qualified variable names (that is, those > > with the module name specified, in this case, the standard module > > Char). You can tell hugs to also load the Char module on top of > > whatever code you have loaded by using the command ":also Char", after > > which the prompt should look like > > Char> > > and you can try > > toUpper 'a' > > Might as well use :load -- in both case Char becomes the new current > module, making the old current module inaccessible. There is a difference, though. Compare Test> :l Char Char> :n my_map ERROR - No names selected and Test> :a Char Char> :n my_map Test.my_map (1 names listed) So with :a(lso) the old module remains in sight for :n(ames) - and for :i in qualified form. Unfortunately I haven't found a way to use them, Hugs (Nov. 2003) in general refuses qualified names, though with some rather weird behaviour: Prelude> :l Test Test> :a Char Char> Prelude.map toUpper "buh" "BUH" Char> Data.Char.toUpper 'u' 'U' Char> :i Test.my_map my_map :: (a -> b) -> [a] -> [b] Char> Data.Char.toUpper 'u' ERROR - Undefined qualified variable "Data.Char.toUpper" Char> Prelude.map toUpper "buh" ERROR - Undefined qualified variable "Prelude.map" How can it be that Hugs knows these qualified names before I ask about Test.my_map but not afterwards? Puzzled, Daniel

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WinHugs Problem
by david 28 Apr '05

28 Apr '05

I have a dell laptop with Novemeber 2003 Hugs installed. The problem is when I run WinHugs in haskell 98 or hugs mode there is no output after an expression is evaluated, there is just a blank line and the interpreter waiting for more input. The problem is only with WinHugs the regular Hugs that runs at the command prompt works just fine. If anyone has any suggestions please let me know.

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No way to fork() ?
by John Goerzen 18 Apr '05

18 Apr '05

Hello, I am trying to write my own pOpen for Haskell. I have a version that works fine in GHC, but I can't quite make it work in Hugs. There are two things missing: the ability to fork and the ability to install a signal handler for SIGPIPE. First, on the forking. There is a vague note on http://cvs.haskell.org/Hugs/pages/libstatus.html about co-operative concurrency. I don't understand why this is relevant. If I want to fork, let me fork; using a low-level function implies that the programmer knows what's going on anyway. I have no intention of using any other sort of concurrency in this program, so there should be no problem. Moreover, Hugs has a call to system(), which internally must use fork() anyway. (I'm puzzled why rawSystem() is missing; somewhere, FFI must have been used to get to system(), right?) The same note applies to signals, and again, I'm not sure why. Also, Hugs doesn't seem to have System.Process, though there's no note explaining why it is not present. I'm probably missing something important here, but I can't figure out what it is. Lacking these functions really diminishes Hugs' usefulness as a scripting language to me, which is a shame; it would make a great one. Thanks, -- John

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Re: [Hugs-users] overloading operators
by Richard A. O'Keefe 13 Apr '05

13 Apr '05

Alfredo Paz-Valderrama <apaz(a)star.com.pe> wrote: but whe the Thompson's book says: -------------------------------------------- Literals are not overloaded, Here is what the Haskell 98 report says, in chapter 3 on page 18: An integer literal represents the application of the function fromInteger to the appropriate value of type Integer. Similarly, a floating point literal stands for an application of fromRational to a value of type Rational (that is, Ratio Integer). +----------------------------------------------------------------+ |Translation: The integer literal i is equivalent to | |fromInteger i, where fromInteger is a method in class Num (see | |section 6.4.1). | |The floating point literal f is equivalent to fromRational (n | |Ratio.% d), where fromRational is a method in class Fractional | |and Ratio.% constructs a rational from two integers, as defined | |in the Ratio library. The integers n and d are chosen so that | |n/d = f. | +----------------------------------------------------------------+ In short, the Report clearly and explicitly says that literals *ARE* overloaded! there is no automatic conversion from Int to Float. But this is true. There is no automatic conversion from Int to Float *EXCEPT* for literals. .. we will receive an error message if we type (floor 5.6) + 6.7 note that the type of (floor 5.6) is not Int and the type of 6.7 is not Float. What we have is 5.6 :: Fractional a => a floor :: (RealFrac a, Integral b) => a -> b Now RealFrac is a sub-type-class of Real and Fractional, so Fractional a & RealFrac a => RealFrac a and 5.6 must belong to some instance of RealFrac. Here's where defaults come into play. The Haskell98 report says in chapter 4 on page 52 that If no default declaration is given in a module, then it is assumed to be default (Integer, Double) Basically, the default declaration says "if you can't figure out what some type variable is, and it must be an instance of at least one of the built-in numeric typeclasses, then pick the first type in this list that works." So in the absence of any 'default' decalaration of your own, (floor 5.6) is really ((floor :: Integral b => Double -> b) ((fromRational :: Ratio Integer -> Double) (Ratio.% (56 :: Integer) (10 :: Integer)))) :: b and 5.6 counts as a Double (***not*** a Float). floor 5.6 :: Integral b => b 6.7 :: Fractional c => c (+) :: Num d => d -> d -> d Now we have d = b and d = c, so we are looking for (+) :: Num d, Integral d, Fractional d => d -> d -> d But there is no type d which belongs to both Integral and Fractional. That is, there is NOT a clash between Int and Double, but a clash between "being an instance of Integral" and "being an instance of Fractional". since we are trying to add quantities of two different types. We have to convert the Int to a Float to perform the addtion, thus: As noted above, according to the report, the default type for a floating point literal is Double, not Float. fromIntegral (floor 5.6) + 6.7 where fromInt takes int to corresponding Float. Is fromInt a typo here? There is no 'fromInt' in the Haskell report. sadly, if I use a integer number it's work! 5 + 6.7 What's sad about that? Wouldn't you want it to? WHY????? Because literals *are* overloaded. In *both* cases you need a call to fromInteger to get the right type, but when it's an integer literal the compiler inserts the call to fromInteger for you. 5 is don't Integer like (floor 5.6)? 5 belongs to *every* numeric type. (floor 5.6) belongs to *every* integral type. So 5 belongs to every type that (floor 5.6) does AND MORE.

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Re: [Hugs-users] overloading operators
by Richard A. O'Keefe 13 Apr '05

13 Apr '05

Alfredo Paz-Valderrama <apaz(a)star.com.pe> is confused by / in Haskell. 1 / 2 I get 0.5 by answer, but the / operator signature is: float -> float -> float No, it certainly is NOT that. At the Hugs prompt, type :type (/) and you will see the answer (/) :: Fractional a => a -> a -> a That is, for any type "a" which belongs to the Fractional type-class, there is a version of (/) which takes two arguments of type "a" and returns a value of type "a". 1) Why don't fail? Because the definition of Haskell says it mustn't. Do read a good tutorial, or textbook, or the report. 2) 1 and 2 are not integers? Not exactly. Or more precisely, not _just_ integers. Again, at the Hugs prompt, type :type 1 and you will see the answer 1 :: Num a => a That is, for any type "a" which belongs to the Num type-class, 1 can be considered as a member of type "a". More precisely, it is as if integer literals like 1 and 2 belonged to the type Integer (unbounded integers) but were always wrapped up as (fromInteger 1) or (fromInteger 2), where fromInteger :: Num a => Integer -> a converts an unbounded integer to whatever numeric type the context requires. A similar rule applies to numeric literals with decimal points. Technically those are exact unbounded rational numbers with fromRational wrapped around them. 3) can this fact be source of confusions? No. Quite the opposite, in fact. Haskell seems to cause much less surprise with arithmetic than any other typed programming language I know. This all relies on typeclasses, which are a fundamental and pervasive part of Haskell.

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overloading operators
by Alfredo Paz-Valderrama 13 Apr '05

13 Apr '05

Hello I am newly in functional programming and haskell too. I have many question about operator's signature. If i write this in hugs: 1 / 2 I get 0.5 by answer, but the / operator signature is: float -> float -> float 1) Why don't fail? 2) 1 and 2 are not integers? 3) can this fact be source of confusions? -- Alfredo Paz-Valderrama Sociedad Peruana de Computacion ---------------------------------------------------------------- This mail sent through StarMail: http://www.star.com.pe

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"Maximum token length (4000) exceeded"
by Ian Lynagh 10 Apr '05

10 Apr '05

Hi, darcs has a file which has a very large string literal in it, so I am getting ERROR "Context.hs":4028 - Maximum token length (4000) exceeded when I try to load it. Is there any chance this could be dynamically allocated so it can grow to accomodate large tokens please? Thanks Ian

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