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Re: Interpret haskell within haskell.
by oleg＠pobox.com 24 Dec '02

24 Dec '02

Lauri Alanko wrote on Dec 22: > The magic is _here_: > (begin (define a 5) (eval '(set! a (+ a 1)) (interaction-environment)) a) > ==> 6 > Here (interaction-enviroment) is a run-time representation of the > compile-time environment. It makes possible two-way interaction between the > stages. Essentially, first-class environments are all the magic you need to > implement eval. I'm sorry to act as a Grinch, but there is no magic. First, according to R5RS, (interaction-environment) is not a first-class environment. R5RS says (Section 6.5): > [[optional procedure]] (interaction-environment) > This procedure returns a specifier for the environment that contains > implementation-defined bindings, typically a superset of those listed > in the report. The intent is that this procedure will return the > environment in which the implementation would evaluate expressions > dynamically typed by the user. There is nothing there about interaction between phases. And in fact, this interaction does not work. If you put the following code (module aaa) (begin (define a 5) (eval '(set! a (+ a 1)) (interaction-environment)) (display a) (newline)) into a file, _compile_ that file using Bigloo, and run the resulting a.out, you will see a message *** ERROR:bigloo:eval: Unbound variable -- a Nowhere the Scheme report talks about first-class environments. Please note that the result of (interaction-environment) is a _specifier_ for an environment. Some Scheme systems do provide first-class environments. For example, MIT Scheme has a form 'the-environment'. It comes at a price. In a post "Re: why isn't the environment explicit ?" on comp.lang.scheme on Mar 15, 2001, Joe Marshall wrote: > > Since you can (sort of) regard call/cc as making continuations > > explicit, why aren't environments explict ? > In MIT Scheme, they are. The special form (the-environment) returns > the current environment as a first-class object. > However. > The difficulty with using such a form is that it destroys any > possibility of compiler optimization. If you compile code with a > (the-environment) form in it (in MIT Scheme), the compiler essentially > throws up its hands and inserts code to call the interpreter. There is no Santa either.

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Re: Interpret haskell within haskell.
by Jonathan.C.Cast-1 24 Dec '02

24 Dec '02

----- Original Message ----- From: Matt Hellige <matt(a)immute.net> Date: Friday, December 20, 2002 0:20 am Subject: Re: Interpret haskell within haskell. > In any case, here are my guesses at why Haskell doesn't have "eval": > - It is difficult to make the kind of run-time safety guarantees > that we are used to with Haskell/ML if we introduce constructs of > this kind. > - It would seem to require considerable typing information to be > carried around at run-time, which in general Haskell compilers > seem to rely on being able to avoid. More generally, it may > render unsafe a wide range of optimizing program transformations. > - It would require the run-time for potentially every program to be > considerably larger, carrying around an entire interpreter. > - It has not, historically, been a priority for the community. The > FP community has tended to focus more on safety and static > analysis, as well as efficiency of compiled output. The general > answer has > been, "Well, if you want that, you should be using > lisp/scheme/etc." > Are those reasons basically right, basically wrong, somewhere in the > middle? If it's actually mostly the final reason, I wonder if the idea > deserves more consideration. Has the idea been revisited in light of > monadic control flow, type classes and other relatively recent > developments? Would it be possible, in this case, to have our cake > and eat it too? Are you trying to argue here that monads, type classes, etc. remove the difficulties you list? Otherwise, it seems to me that supporting eval would remove just about every advantage Haskell has over imperative languages. > Matt > > -- > Matt Hellige matt(a)immute.net > http://matt.immute.net Jon Cast

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Re: Interpret haskell within haskell.
by oleg＠pobox.com 22 Dec '02

22 Dec '02

David J. Sankel wrote: > I was referring to a haskell interpreter to be used > within haskell code. For instance: > main = do > user_configuration <- parseHaskell > title <- resolveFunction user_configuration "title" > :: String > putStr title This was exactly the gist of the message: http://www.haskell.org/pipermail/haskell-cafe/2002-September/003423.html The message gave the working example. The end of the message posed that GHCi is better for these kinds of things. Simon Marlow's message shows that this is indeed the case.

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Fail: <<loop>> ???
by Ingo Wechsung 21 Dec '02

21 Dec '02

Dear Haskellers, when I execute a program made with GHC (standard Haskell only, no optimization) I get Fail: << loop>> I have -W on, but it only tells me about a single overlapping pattern, which is ok. The same program crashes Hugs alltogether. Any hints for what I should look in my sources? MfG Ingo

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Tree insert, lookup with keys
by Ingo Wechsung 20 Dec '02

20 Dec '02

Hi, perhaps I don't understand the type system fully yet: I want a tree that carrys some information of type s; data Tree s = TNil | Branch s Tree Tree; This is fine for String, Integer and other atomic types. ins :: Tree s -> s -> Tree s; ins Tnil s = Branch s Tnil Tnil; ins (Branch s l r) x = case compare x s of { LT -> Branch s (ins l x) r; EQ -> Branch s l r; GT -> Branch s l (ins r x); } So far so good, however, I do not always want to compare everything. What I really want is to have (compare (key x) (key s)) instead in the definition of ins. My attempt: class Keyed a where { -- Type a is keyed if it has a key function. key :: Ord b => a -> b; -- key is a function, that, when applied to a yields some b that is comparable } and then data Sym = Sym String Int Ty; -- Ty is another algebraic type instance Keyed Sym where { key (Sym a _ _) = a; -- use string as key } But I always get the following message: Cannot unify the type-signature variable `b` with the type `String` Expected type: b Inferred Type: String In the definition of `key': a Why is this? Has "key" to return the very same type for every instance? After all, I need type `b' in compare only so it should not matter what it really is. The compiler could check if for every instance key is defined so that the return type is some instance of Ord, couldn't he. Merry Christmas Ingo

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AW: Tree insert, lookup with keys
by Ingo Wechsung 20 Dec '02

20 Dec '02

Ketil wrote: >But it isn't obvious what b is supposed to be. Sure, it must be an instance of Ord. In a way, I promise the compiler not to use anything but compare on b's. Of course, when I define n instances, there will be n different b-Types that must not be confused. >(Requires -fglasgow-exts) Thanks for the tip. Since I am obviously still learning, I prefer not to use the extensions until I know exactly what they are about. (As it seems, I am just experiencing one case where the extensions are useful. I'm sure there are many other cases.) Meanwhile, I see 2 possibilities to solve the problem in standard Haskell: (a) Stick with String keys key :: a -> String; or, (b) forget class Keyed and for every type s in Tree s derive an instance from Ord that defines compare accordingly. Solution (b) seems not so good to me since we might need the standard Ord behaviour elsewhere. Solution (a) is practical and should perform fairly well, except when we must write something like key (Baz x s y) = show x; Are there any other solutions in standard Haskell? Ingo

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Parsing date and time specifications
by Peter Simons 20 Dec '02

20 Dec '02

Hi, I have written a parser that turns an RFC2822 date and time specification into a datatype usable in Haskell. While the parser is working just fine so far, I have a problem with the CalendarTime datatype. It appears that in order to construct one of those, I need _all_ the information it contains, including the weekday (Day) and the number of the day in the year. The problem now is that I do not have this information! Of course I could calculate these values by hand, but this is immensely complicated. I thought about using a TimeDiff instead, but apparently the functions provided in the Prelude do not handle TimeDiff for anything except for "maths" -- what doesn't really help me for what I am trying to do. I briefly looked at the Posix module that comes with GHC as well, and which seems to provide CTime. But then, the Posix module appears to be non-standard. *sigh* Any suggestions what I could do? -peter

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Re: Library for concurrent threads running
by Kevin Hammond 18 Dec '02

18 Dec '02

I am also not quite sure of your motivation! The current version of GpH is (or can be with very little difficulty) implemented using library calls. There is really no language extension per se, rather there are calls to tightly-coupled C code, plus a set of high level libraries to provide structure (evaluation strategies). There are, of course, modifications to the RTS to support proper scheduling, communications etc. , but these are necessary for a properly integrated high-level approach. If you want a low-level approach, one of the MPI libraries for Haskell may suit you better? Kevin At 7:16 pm +0100 16/12/02, Christian Sievers wrote: Dusan Kolar asked: > My question/wish is maybe naive, but anyway: > Is there a library (not a language extension, like > Concurrent Haskell, Glasgow Parallel Haskell, ...) enabling > to run two functions in parallel, possibly in cascade? I think the best you can get without language extensions is what is described in Koen Claessen's Functional Pearl "A Poor Man's Concurrency Monad". It's really nice, but not exactly what you seem to want. You can get that article from http://www.cs.chalmers.se/~koen/publications.html All the best Christian Sievers _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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Random
by Filipe Santos 18 Dec '02

18 Dec '02

I need some help to do a function so that I cant get 4 numbers between 1 and 6, i tried to use random but i can't make it work well.

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(no subject)
by Olga Lucia Quintero Fonseca 18 Dec '02

18 Dec '02

Por favor, estoy aprendiendo el lenguaje y no se como usar Complex aqui, el prompt siempre esta como Prelude y no se como incluir ese package module. Podrian ayudarme ? De antemano gracias !!!

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