Re: [Haskell] question about a failure to generalize
Quoting Stefan O'Rear <[EMAIL PROTECTED]>: On Mon, Sep 17, 2007 at 04:15:10PM +1200, Tom Pledger wrote: Norman Ramsey wrote: : | This code fails to compile because the compiler is willing to | use 'fold' at only one type (CmmExpr as it happens) : When it failed to compile, was fold = foldRegsUsed a top-level declaration in the module, rather than local to foldRegsUsed? If so, try working around the monomorphism restriction by changing from a pattern binding to a function binding. fold f = foldRegsUsed f The monomorphism restriction is not affected by top-level-or-not, see sections 4.5.1 and 4.5.5 in the Haskell 98 Language and Libraries Report. Stefan Aargh! Sorry, I'm having a Bad Details Day. 3 in 1 message: - Overlooking the "to make the code work, I had to expand 'fold' into 'foldRegsUsed' everywhere it appears" part in the original question, - Chopping a letter out of Norman's email address, and - Mixing up the MR with other issues, possibly something from the binding groups section of Typing Haskell In Haskell. The one saving grace is that I tested my suggestion before posting, and it worked. :-) - Tom ___ Haskell mailing list Haskell@haskell.org http://www.haskell.org/mailman/listinfo/haskell
Re: [Haskell] question about a failure to generalize
On Mon, Sep 17, 2007 at 04:15:10PM +1200, Tom Pledger wrote: > Norman Ramsey wrote: > : > | This code fails to compile because the compiler is willing to > | use 'fold' at only one type (CmmExpr as it happens) > : > > When it failed to compile, was > > fold = foldRegsUsed > > a top-level declaration in the module, rather than local to foldRegsUsed? > > If so, try working around the monomorphism restriction by changing from a > pattern binding to a function binding. > > fold f = foldRegsUsed f The monomorphism restriction is not affected by top-level-or-not, see sections 4.5.1 and 4.5.5 in the Haskell 98 Language and Libraries Report. Stefan signature.asc Description: Digital signature ___ Haskell mailing list Haskell@haskell.org http://www.haskell.org/mailman/listinfo/haskell
[Haskell] question about a failure to generalize
Norman Ramsey wrote: : | This code fails to compile because the compiler is willing to | use 'fold' at only one type (CmmExpr as it happens) : When it failed to compile, was fold = foldRegsUsed a top-level declaration in the module, rather than local to foldRegsUsed? If so, try working around the monomorphism restriction by changing from a pattern binding to a function binding. fold f = foldRegsUsed f Regards, Tom ___ Haskell mailing list Haskell@haskell.org http://www.haskell.org/mailman/listinfo/haskell
Re: [Haskell] Swapping parameters and type classes
On Sun, Sep 16, 2007 at 10:45:39PM +0200, Bas van Dijk wrote:
> On 9/16/07, Mads Lindstrøm <[EMAIL PROTECTED]> wrote:
> > But what if I want to "apply" the 'b' ? How do I do that ?
>
> The following uses type families (functions) and compiles under GHC HEAD:
>
> {-# OPTIONS_GHC -XTypeFamilies -XEmptyDataDecls -XTypeSynonymInstances #-}
Eek!
That should be:
{-# LANGUAGE TypeFamilies, EmptyDataDecls, TypeSynonymInstances #-}
Modulo the fact that only GHC support type families at the moment, the
latter will be portable...
(Ian/Simon: I've seen this several times now. Maybe there should be a
warning for -X in OPTIONS? Is that even feasable?)
Stefan
signature.asc
Description: Digital signature
___
Haskell mailing list
Haskell@haskell.org
http://www.haskell.org/mailman/listinfo/haskell
Re: [Haskell] Swapping parameters and type classes
On 9/16/07, Bas van Dijk <[EMAIL PROTECTED]> wrote: > The following uses type families (functions) and compiles under GHC HEAD: > ... Oops this is not correct! Its getting late... oh well Bas ___ Haskell mailing list Haskell@haskell.org http://www.haskell.org/mailman/listinfo/haskell
Re: [Haskell] Swapping parameters and type classes
On 9/16/07, Mads Lindstrøm <[EMAIL PROTECTED]> wrote:
> But what if I want to "apply" the 'b' ? How do I do that ?
The following uses type families (functions) and compiles under GHC HEAD:
{-# OPTIONS_GHC -XTypeFamilies -XEmptyDataDecls -XTypeSynonymInstances #-}
data Foo a b
class Bar (x :: * -> *)
instance Bar (Foo a)
type family BarB a b :: * -> *
type instance BarB a b = Foo b
instance Bar (BarB a b)
regards,
Bas van Dijk
___
Haskell mailing list
Haskell@haskell.org
http://www.haskell.org/mailman/listinfo/haskell
Re: [Haskell] Swapping parameters and type classes
On 9/16/07, Mads Lindstrøm <[EMAIL PROTECTED]> wrote: > > Hi all > > If I have this type: > > data Foo a b = ... > > and this class > > class Bar (x :: * -> *) where ... > > I can imagine two ways to make Foo an instance of Bar. Either I must > "apply" the 'a' or the 'b' in (Foo a b). Otherwise it will not have the > right kind. To "apply" the 'a' I can do: > > instance Bar (Foo a) where ... > > But what if I want to "apply" the 'b' ? How do I do that ? One easy way would be to create a newtype with the type parameters swapped: newtype Oof b a = Oof (Foo a b) instance Bar (Oof b) where ... Of course, if you want to partially apply the second parameter of a function, you use 'flip'. I thought for a while about whether there's some sort of typeclass hackery which is directly parallel to the use of 'flip', but couldn't come up with anything. Anyone? -Brent ___ Haskell mailing list Haskell@haskell.org http://www.haskell.org/mailman/listinfo/haskell
Re: [Haskell] HaL2: Meeting Haskell in Leipzig 2, videos and presentations online
Am Sonntag, 16. September 2007 14:22 schrieb klaus meier: > Hello haskell friends! > > The Videos and presentations of the talks given at HaL2 are now online: > […] > 2 Wolfgang Jeltsch (TU Cottbus) talks about "Grapefruit, a Haskell- > library for the declarative description of graphic user interfaces" I’d like to add that this video doesn’t cover the complete talk but only the first five minutes. :-( Alf, could you please make sure that this gets corrected? Thanks a lot! > […] Best wishes, Wolfgang ___ Haskell mailing list Haskell@haskell.org http://www.haskell.org/mailman/listinfo/haskell
[Haskell] Records
Now that I have a version of ghc with type classes, I have had a go at implementing records based on the ideas I mentioned on this list a few months ago. The code of my first attempt is available at http:// homepage.ntlworld.com/b.hilken/files/Records.hs I am releasing this to get feedback. I think Haskell needs a records system of this kind of generality, and this code at least allows you to play around. From the comment section of the file: --- Record construction: EmptyRecis the empty record. N =: x is the record with one field labelled N carrying data x. t +: u is the union of records t and u. Any overlap of labels gives a static error. Record destruction: t .: N is the value of field N in record t. A lack of field N gives a static error. t -: N is record t with field N deleted. A lack of field N gives a static error. Record update: t |: u is the record with fields from u where it has them, t otherwise. If u has any fields not in t, or of different types from t, there is a static error. Note that the result has the same type as t. All these records have types: EmptyRecis the type of the empty record. N :=: a is the type of a record with one field labelled N carrying data of type a. r :+: s is the union of record types r and s. Any overlap of labels gives a static error. r :.: N is the type of field N in a record of type r. A lack of field N gives a static error. r :-: N is record type r with field N deleted. A lack of field N gives a static error. Finally some classes to govern the polymorphism: r `Contains` N means that r is a record type with a field labelled N. r `Disjoint` s means that r and s are record types with no fields in common. r `Subrecord` s means that r and s are record types, and every field of r also occurs in s (with the same type). The types of the basic operators are as follows: (=:) :: n -> a -> n :=: a (+:) :: r `Disjoint` s => r -> s -> r :+: s (.:) :: r `Contains` n => r -> n -> r :.: n (-:) :: r `Contains` n => r -> n -> r :-: n (|:) :: r `Subrecord` s => s -> r -> s -- Note that these records are a lot more expressive than the Hugs system, as you can not only extend records by adding fields, but also take unions of arbitrary (disjoint) records. Record update is designed for functions with lots of named optional arguments. If you define f opts = ... options.:Optj ... where options = (Opt1 =: val1 +: ... +: Optn =: valn) |: opts then the user can write (for example): f (Optk =: u +: Optl =: v) to set just two of the options, leaving the rest as default. This also cannot be done in the Hugs system. The main disadvantage of the current implementation is that you have to tell the compiler in which order to store the fields, by defining one of the following: type instance NameCmp N M = NameLT type instance NameCmp N N = NameEQ type instance NameCmp N M = NameGT for each pair of labels N & M in such a way as to give a linear order on labels. You need n^2 definitions, where n is the number of labels. I would do this in Template Haskell, but it won't yet allow you to declare type instances. Maybe some compiler support? Error messages tend to be cryptic. They mostly complain of missing instances, and can run to several pages. There is really no way to improve this without building it all in to the compiler! All comments gratefully received, including suggestions on syntax, choice of operators, implementation, explanation, etc. Barney. ___ Haskell mailing list Haskell@haskell.org http://www.haskell.org/mailman/listinfo/haskell
[Haskell] Swapping parameters and type classes
Hi all If I have this type: data Foo a b = ... and this class class Bar (x :: * -> *) where ... I can imagine two ways to make Foo an instance of Bar. Either I must "apply" the 'a' or the 'b' in (Foo a b). Otherwise it will not have the right kind. To "apply" the 'a' I can do: instance Bar (Foo a) where ... But what if I want to "apply" the 'b' ? How do I do that ? Greetings, Mads Lindstrøm ___ Haskell mailing list Haskell@haskell.org http://www.haskell.org/mailman/listinfo/haskell
[Haskell] HaL2: Meeting Haskell in Leipzig 2, videos and presentations online
Hello haskell friends! The Videos and presentations of the talks given at HaL2 are now online: http://iba-cg.de/haskell.html Meeting HaL2, Leipzig Germany 10th of July Talks: (first in english, remaining in german) 1 Johan Jeuring (Uni Uetrecht) talks about "Generic Programming in Haskell" 2 Wolfgang Jeltsch (TU Cottbus) talks about "Grapefruit, a Haskell- library for the declarative description of graphic user interfaces" 3 Henning Thielemann (Uni Halle) talks about "Calculation of sound in Haskell" 4 Leif Frenzel (Karlsruhe) introduced Cohatoe: "Writing eclipse- plugins in haskell" We hope to see you on the next HaL meeting in the first quarter of 2008. Greetings, Alf Richter ___ Haskell mailing list Haskell@haskell.org http://www.haskell.org/mailman/listinfo/haskell
