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List types
by Eric Allen Wohlstadter 04 Dec '00

04 Dec '00

Is there a way to make a list that contains multiple types? If not, why, and isn't this a serious restriction? I would like to be able to say: map show ['a',5] Since both Char and Num derive Show it seems like a reasonable thing to do. I realise classes in haskell don't work this way so I guess my question is, why not? Could I do something like this in O'Haskell, Clean, or some other functional language? In particular I am interested in exploring software engineering implications of monadic IO, separation of concerns, etc... Eric Wohlstadter UCDavis Software Engineering

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RE: Beginner: error when using multiple where stmts in hugs98
by Mark P Jones 01 Dec '00

01 Dec '00

| I've been trying some examples in functional programming. Most things | work fine, but I have trouble with expressions with 'where' clauses | that define more then one local definition. | ... | For example: | ... | mydiff f = f' | where f' x = ( f (x+h) - f x) / h | h = 0.0001 The second line in the where clause (your attempt to define local variable h) will be treated as if it were part of the first definition (for the variable f'). Why? Because that's what the layout/indentation in your code specifies. In fact, to a Haskell compiler, your input looks a lot like the following, clearly incorrect definition: mydiff f = f' where f' x = ( f (x+h) - f x) / h h = 0.0001 If you want h to be defined as a local variable at the same level as f', then you should make sure the two definitions start with the same indentation: mydiff f = f' where f' x = ( f (x+h) - f x) / h h = 0.0001 The indentation you've actually used suggests that you might have intended the definition of h to be local to f'. In that case, you should add an extra "where" keyword: mydiff f = f' where f' x = ( f (x+h) - f x) / h where h = 0.0001 All the best, Mark

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RE: a trap for the unwary
by Mark P Jones 01 Dec '00

01 Dec '00

Hi Malcolm, | Today, I thought I had discovered a bug in ghc. Then I tried hbc | and Hugs, and they also rejected my program with the same error. | nhc98 alone accepts it without complaint. I looked up the Report, | and it seems that the program is indeed incorrect. | | > f x = y | > where | > y | isSpace x = True | > y | otherwise = False For what it's worth, older versions of Hugs would have accepted this too. Only quite recently was it changed to conform to the report. Incidentally, although the above is not valid Haskell, the following should work just fine: f x = y where y | isSpace x = True | otherwise = False The difference is that I haven't repeated the left hand side of the definition in the second equation, which means that it's treated as one definition, not two. All the best, Mark (PS. An even nicer way to write this definition: f = isSpace :-)

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Type classes diagram?
by Kurt 29 Nov '00

29 Nov '00

Has anyone seen a downloadable diagram of the relationships between all of the Haskell type classes? If so, could you post a link? TIA, Kurt.

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RE: Will Haskell be commercialized in the future?
by Doug Ransom 29 Nov '00

29 Nov '00

Development tools for OO now are as good as smalltalk-80. I expect the benefits of FP to be widely adopted in industry by 2020. XSLT is kind of cool and taking off. Not exactly functional but there is no destructive assignment. A lot of the things Haskell excells at (IMO) inferior tools are being used in place. For example, microsoft has build good XSLT translators and two new compilers (C#, VB7) in the last couple years. Unfortunately, C#, not Haskell, will probably be "the" language for the next decade. Fortunately, C++ will not be the language of the next decade. > -----Original Message----- > From: Christian Lescher [mailto:christian@lescher.de] > Sent: Thursday, November 23, 2000 10:14 PM > To: haskell-cafe(a)haskell.org > Subject: Will Haskell be commercialized in the future? > > > In my opinion there are many more real world problems, that > can be most > efficiently solved with functional languages like Haskell, as > (software) > industry can think of at the moment; they only know their C/C++, Java, > etc. but can't even think of the power of functional programming or at > least don't take languages like Haskell for full. (Of corse, there are > exceptions to the rule, too.) > > What do you think: Will Haskell (the related compilers/tools) be > "commercialized" in the future? > > Christian > > > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

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Re: Will Haskell be commercialized in the future?
by Ashley Yakeley 27 Nov '00

27 Nov '00

At 2000-11-27 19:52, Tyson Dowd wrote: >On 27-Nov-2000, Adrian Hey <ahey(a)iee.org> wrote: >> On Mon 27 Nov, Fergus Henderson wrote: >> > Do you think that Haskell would be better without `unsafePerformIO'? >> >> Well, a sceptic like me is bound to wonder why such a non-function is >> provided in a purely functional language. What really worries me is >> that the damage isn't localised. If you allow such things you can never be >> sure that any function really is a function, without careful scrutiny of all >> the code it's dependent on. > >This is an issue, but it arises in any "pure" language with a foreign >language interface. Not necessarily, the functions just need to be typed correctly. In the case of a non-safe or imperative function, that's going to be of the form "a -> IO b" (or "IO a -> IO b" if you prefer the more powerful arrow model). No safety needs to be sacrificed. -- Ashley Yakeley, Seattle WA

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unsubscribe haskell-cafe
by Wooseok Yang 25 Nov '00

25 Nov '00

unsubscribe haskell-cafe

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Will Haskell be commercialized in the future?
by Christian Lescher 24 Nov '00

24 Nov '00

In my opinion there are many more real world problems, that can be most efficiently solved with functional languages like Haskell, as (software) industry can think of at the moment; they only know their C/C++, Java, etc. but can't even think of the power of functional programming or at least don't take languages like Haskell for full. (Of corse, there are exceptions to the rule, too.) What do you think: Will Haskell (the related compilers/tools) be "commercialized" in the future? Christian

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(no subject)
by Chris Angus 24 Nov '00

24 Nov '00

I was wondering if Has anybody had access to a Haskell -> Java compiler/translator. ----------------------------------------- Chris Angus Armature Enterprise House 1 Apex View Leeds LS11 9BH +44 113 2595253 cangus(a)armature.com

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Re: Will Haskell be commercialized in the future?
by Friedrich Dominicus 24 Nov '00

24 Nov '00

Christian Lescher <christian(a)lescher.de> writes: > In my opinion there are many more real world problems, that can be most > efficiently solved with functional languages like Haskell, as (software) > industry can think of at the moment; they only know their C/C++, Java, > etc. but can't even think of the power of functional programming or at > least don't take languages like Haskell for full. (Of corse, there are > exceptions to the rule, too.) > > What do you think: Will Haskell (the related compilers/tools) be > "commercialized" in the future? Let me counter with another question. Do you think people will pay for it and enough to keep the vendors going? Regards Friedrich

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