[Haskell-cafe] Compilation issue with GHC 6.10.1 on Vista
I'm trying to update FreshLib <http://homepages.inf.ed.ac.uk/jcheney/programs/freshlib/> to work with the current GHC version. Now it works fine in GHCi, but trying to compile with ghc --make gives this result: [1 of 5] Compiling NominalBase ( NominalBase.hs, NominalBase.o ) Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. Loading package syb ... linking ... done. Loading package base-3.0.3.0 ... linking ... done. Loading package array-0.2.0.0 ... linking ... done. Loading package packedstring-0.1.0.1 ... linking ... done. Loading package containers-0.2.0.0 ... linking ... done. Loading package pretty-1.0.1.0 ... linking ... done. Loading package template-haskell ... linking ... done. Loading package bytestring-0.9.1.4 ... linking ... done. Loading package syb-with-class-0.4 ... linking ... done. C:\Users\Alexey\AppData\Local\Temp\/ghc1892_0/ghc1892_0.s: Assembler messages: C:\Users\Alexey\AppData\Local\Temp\/ghc1892_0/ghc1892_0.s:557: Error: symbol `_N ominalBase_zdgtoName_closure' is already defined C:\Users\Alexey\AppData\Local\Temp\/ghc1892_0/ghc1892_0.s:585: Error: symbol `_N ominalBase_zdgtoName_info' is already defined C:\Users\Alexey\AppData\Local\Temp\/ghc1892_0/ghc1892_0.s:637: Error: symbol `_N ominalBase_zdgfromName_closure' is already defined C:\Users\Alexey\AppData\Local\Temp\/ghc1892_0/ghc1892_0.s:665: Error: symbol `_N ominalBase_zdgfromName_info' is already defined Yours, Alexey Romanov ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] What I wish someone had told me...
On Tue, Oct 14, 2008 at 9:15 PM, John Lato <[EMAIL PROTECTED]> wrote: > Are you advocating introducing existential types to beginning > Haskellers? I think something with the scary name "existential > quantification" would greatly increase the head'splodin' on the > learnin' slope. Certainly there's a place for them, but I wouldn't > want to see new Haskell programmers habitually approach problems with > a "first create a type class, then make an existential wrapper" > mentality. Which is exactly what I fear is the current situation. I don't think this is the current situation at all. For one, the beginning Haskellers do _not_ learn about existential types. Yours, Alexey Romanov ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] What I wish someone had told me...
On Tue, Oct 14, 2008 at 9:14 PM, Jonathan Cast <[EMAIL PROTECTED]> wrote: > On Tue, 2008-10-14 at 18:15 +0100, John Lato wrote: >> Are you advocating introducing existential types to beginning >> Haskellers? I think something with the scary name > > Invalid argument. > >> "existential >> quantification" would greatly increase the head'splodin' on the >> learnin' slope. > > Invalid argument. Head explosion is the *goal* of teaching Haskell. Is it? I would certainly prefer my students to say "This is obvious. Why would things work in any other way?" They don't, but I can dream. >> Certainly there's a place for them, but I wouldn't >> want to see new Haskell programmers habitually approach problems with >> a "first create a type class, then make an existential wrapper" >> mentality. > > Of course not. That's just translating OO into Haskell. Personally, I > would avoid comparing Haskell to other language at all (SOE I believe > takes this approach). > > jcc I find such comparisons pretty useful. Yours, Alexey Romanov ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] What I wish someone had told me...
Well, they act like interfaces in argument types, just not variable or
return types.
Yours, Alexey Romanov
On Tue, Oct 14, 2008 at 4:11 PM, John Lato <[EMAIL PROTECTED]> wrote:
> I was just thinking about what I wish someone had told me when I
> started working with Haskell (not that long ago). It would have saved
> me a great deal of trouble.
>
> The Difference Between Interfaces and Type Classes.
>
> Many "Introduction to Haskell for the OOper" style tutorials claim
> that type classes are like Interfaces (for languages with that
> feature). I now think that, although this is technically true, it's
> fundamentally misleading. Here's a simple example demonstrating my
> line of thought:
>
> In C# (and presumably Java), this sort of function is common:
> public IList GetListOfData()
>
> In Haskell, a similar function may be
> GetListOfData :: (Foldable a, Indexable a) => IO a
>
> In C#, it doesn't matter what the actual type returned by
> GetListOfData is, as long is it supports the IList interface. It's
> not uncommon for GetListOfData to make a choice between several
> different implementations, depending on the nature of the data to be
> returned. The following code is perfectly reasonable in C# :
>
> // List and MyList are different classes
> if (something) { return new List(); }
> else { return new MyList(); }
>
> The equivalent won't compile in Haskell, because the actual return
> type does matter, and *is determined by the calling code*. Our
> fictional GetListOfData can't return a List or a Mylist depending on
> some conditional, in fact it can't explicitly return either one at
> all, because the actual type of the result, as determined by the
> caller, could be either one, or something else entirely (ignoring the
> IO bit for the time being).
>
> So I've come to the conclusion that stating type classes are like
> interfaces is misleading to newcomers to Haskell, because it leads
> people to think they should use type classes for the same sorts of
> problems OO-languages solve with interfaces. In turn this means new
> programmers are encouraged to use OO-style architecture in programs,
> reassured that they're in a "functional" idiom because they're using
> the "functionally-approved" feature of type classes. I think that if
> I had understood this earlier, I would have embraced functional idioms
> more quickly.
>
> Incidentally, I'm still horrible at designing functional APIs and
> modules, but at least now I know it, and I'm no longer trying to force
> OO interfaces into Haskell. So I've made progress.
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