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RE: 'Forall' Polymorphism Question
by Andrew Kennedy 02 Nov '01

02 Nov '01

Not quite. "A definition is impredicative if it refers to a collection which contains the object to be defined." (see http://www.xrefer.com/entry/552390) If you stratify a type system so that quantifiers only quantify over things at a lower "stratum" then (I think) it is predicative (= not impredicative). But you might have forall quantifiers at more than one level in your system, as is the case in some formalizations of ML's module system (Shao et al) where a predicative subset of the impredicative F_\omega is used. I'm also willing to be corrected! - Andrew. > -----Original Message----- > From: Simon Peyton-Jones [mailto:simonpj@microsoft.com] > Sent: Wednesday, October 31, 2001 12:20 PM > To: Ashley Yakeley; Haskell List > Subject: RE: 'Forall' Polymorphism Question > > > Any system that allows a parametric type variable to > be instantiated by a for-all type is called "impredicative", > I think. Example: Maybe (forall a. a->a), or as you have > below (m (forall a. a->a)). > > I don't understand all (well, any) of the details, but I > understand that impredicative systems make type inference > nigh impossible. I don't know any programming language that > has impredicative types. Haskell certainly does not, nor any > extension I know of. > > I'm willing to be corrected! > > Simon > > | -----Original Message----- > | From: Ashley Yakeley [mailto:ashley@semantic.org] > | Sent: 31 October 2001 10:14 > | To: Haskell List > | Subject: 'Forall' Polymorphism Question > | > | > | I note that GHC by default gives this type for "return id": > | > | (return id) :: forall m a. (Monad m) => m (a -> a) > | > | Wouldn't this be more general: > | > | (return id) :: forall m. (Monad m) => m (forall a. a -> a) > | (return return) :: forall m. (Monad m) => m (forall m1 a. > | (Monad m1) > | => a -> m1 a) > | > | ...? > | > | Is this something GHC could ever do, or are there good > reasons why it > | would never work in Haskell? > | > | -- > | Ashley Yakeley, Seattle WA > | > | > | _______________________________________________ > | Haskell mailing list > | Haskell(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell > | > > _______________________________________________ > Haskell mailing list > Haskell(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell >

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Transmitting parameters
by raul sierra alcocer 02 Nov '01

02 Nov '01

Hi, What mechanism of transmiting parameters does Haskell implement? Thank you, Raul

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Re: Transmitting parameters
by Lennart Augustsson 02 Nov '01

02 Nov '01

> >> >What mechanism of transmiting parameters does Haskell implement? > >> > >> By value. > > > >Yes, though one might equally say that they are passed by > >reference, since in > > > > Oh, yeah, they're passed by thunk. I forgot about the laziness. Well, Haskell does not really specify which mechanism to use. Most implementations use call-by-need (a variation of the old call-by-name). But any mechanism that is faithful to the semantics would, in principle, be all right. So call-by-name is all right. As is spawning a thread for the parameter (I don't know the name of this mechanism), as in pH. -- Lennart

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ANNOUNCE: Generic Haskell version 0.99 (Amber)
by Andres Loeh 02 Nov '01

02 Nov '01

Generic Haskell version 0.99 (Amber) ==================================== We are happy to announce the first release of Generic Haskell, an extension of Haskell which facilitates generic programming. Generic Haskell includes the following features: * type-indexed values -- generic functions definable for all Haskell data types. * kind-indexed types -- for giving types to type-indexed values * type-indexed types -- types which are indexed over the type constructors underlying Haskell datatypes. The Generic Haskell compiler takes Generic Haskell source and produces Haskell code (which is in most cases Haskell 98 compliant). Download -------- The Generic Haskell compiler is available in source and binary distributions. Binaries for Linux, Windows, MacOSX, and Solaris are available. These are available from: http://www.generic-haskell.org/compiler.html The documentation is also available separately from that page. For more general information, point your browser to: http://www.generic-haskell.org Why Generic Haskell? -------------------- Software development often consists of designing datatypes, around which functionality is added. Some functionality is datatype specific, whereas other functionality is defined on almost all datatypes in such a way that it depends only on the structure of the datatype. A function that works on many datatypes in this way is called a generic function. Examples of generic functionality include storing a value in a database, editing a value, comparing two values for equality, pretty-printing a value. Since datatypes often change and new datatypes are introduced, we have developed Generic Haskell, an extension of the functional programming language Haskell that supports generic definitions, to save the programmer from (re)writing instances of generic functions. The design of Generic Haskell is based on recent work by Ralf Hinze. Pleasant programming, The Generic Haskell Team at Utrecht University (Dave Clarke, Ralf Hinze, Johan Jeuring, Andres Loeh, Jan de Wit) info(a)generic-haskell.org

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Re: Transmitting parameters
by Ashley Yakeley 02 Nov '01

02 Nov '01

At 2001-11-02 02:21, Jon Fairbairn wrote: >> >What mechanism of transmiting parameters does Haskell implement? >> >> By value. > >Yes, though one might equally say that they are passed by >reference, since in > >g = let f x = x+x > z = factorial 1000 > in f z * z > >the 'first' instance of x forces z to be evaluated and >updated, the second instance of x uses this updated value, >and so does the final occurrence of z. Oh, yeah, they're passed by thunk. I forgot about the laziness. -- Ashley Yakeley, Seattle WA

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Re: Transmitting parameters
by Ashley Yakeley 02 Nov '01

02 Nov '01

At 2001-11-01 22:10, raul sierra alcocer wrote: >What mechanism of transmiting parameters does Haskell implement? By value. -- Ashley Yakeley, Seattle WA

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RE: 'Forall' Polymorphism Question
by Simon Peyton-Jones 31 Oct '01

31 Oct '01

Any system that allows a parametric type variable to be instantiated by a for-all type is called "impredicative", I think. Example: Maybe (forall a. a->a), or as you have below (m (forall a. a->a)). I don't understand all (well, any) of the details, but I understand that impredicative systems make type inference nigh impossible. I don't know any programming language that has impredicative types. Haskell certainly does not, nor any extension I know of. I'm willing to be corrected! Simon | -----Original Message----- | From: Ashley Yakeley [mailto:ashley@semantic.org] | Sent: 31 October 2001 10:14 | To: Haskell List | Subject: 'Forall' Polymorphism Question | | | I note that GHC by default gives this type for "return id": | | (return id) :: forall m a. (Monad m) => m (a -> a) | | Wouldn't this be more general: | | (return id) :: forall m. (Monad m) => m (forall a. a -> a) | (return return) :: forall m. (Monad m) => m (forall m1 a. | (Monad m1) | => a -> m1 a) | | ...? | | Is this something GHC could ever do, or are there good reasons why it | would never work in Haskell? | | -- | Ashley Yakeley, Seattle WA | | | _______________________________________________ | Haskell mailing list | Haskell(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell |

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varying number of arguments restriction
by Hal Daume 31 Oct '01

31 Oct '01

I'm curious why the following code is invalid (from a language design point of view): > foo :: [(Int, String)] -> String > foo [] = "" > foo = snd . head ghc complains: Varying number of arguments for function `foo' I don't understand why this should be invalid? Basically, as I read it, I say, well, for a parameter s, calling foo s: does s match [] ? if so, return "" otherwise, try the next definition of foo (which must match in this case). obviously i can rewrite: foo [] = "" foo s = (snd . head) s but this is uglier. - hal -- Hal Daume III "Computer science is no more about computers | hdaume(a)isi.edu than astronomy is about telescopes." -Dijkstra | www.isi.edu/~hdaume

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'Forall' Polymorphism Question
by Ashley Yakeley 31 Oct '01

31 Oct '01

I note that GHC by default gives this type for "return id": (return id) :: forall m a. (Monad m) => m (a -> a) Wouldn't this be more general: (return id) :: forall m. (Monad m) => m (forall a. a -> a) (return return) :: forall m. (Monad m) => m (forall m1 a. (Monad m1) => a -> m1 a) ...? Is this something GHC could ever do, or are there good reasons why it would never work in Haskell? -- Ashley Yakeley, Seattle WA

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Re: varying number of arguments restriction
by Ashley Yakeley 31 Oct '01

31 Oct '01

At 2001-10-30 11:01, Hal Daume wrote: >obviously i can rewrite: > >foo [] = "" >foo s = (snd . head) s > >but this is uglier. I'm not sure. I actually prefer it written out so that the number of arguments in the cases matches (as GHC enforces). -- Ashley Yakeley, Seattle WA

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