- Glasgow-haskell-users

RE: efficiency of UArray
by Simon Peyton-Jones 17 May '02

17 May '02

GHC doesn't remove intermediate lists down both branches of a zip, so yes, you'll get intermediate lists. Why not use array indexing, as per your second version (only in Haskell)? Simon | -----Original Message----- | From: Hal Daume III [mailto:hdaume@ISI.EDU] | Sent: 16 May 2002 00:55 | To: GHC Users Mailing List | Subject: efficiency of UArray | | | can we expect a function like: | | sum [x*y | (x,y) <- zip (elems v) (elems u)] | | to be as efficient as, say: | | sum = 0 | for i=1, n | sum = sum + v[i] * u[i] | | ? | | Basically, will any intermediate lists be created here? | | -- | Hal Daume III | | "Computer science is no more about computers | hdaume(a)isi.edu | than astronomy is about telescopes." -Dijkstra | www.isi.edu/~hdaume | | _______________________________________________ | Glasgow-haskell-users mailing list | Glasgow-haskell-users(a)haskell.org | http://www.haskell.org/mailman/listinfo/glasgow-| haskell-users |

3 2

-fallow-overlapping-instances Request
by Ashley Yakeley 17 May '02

17 May '02

Currently -fallow-overlapping-instances only allows overlapping instances if one is a strict subset of the other. This is good (determinate), but sometimes you really need two instances that partially overlap. It would be nice if this could be disambiguated simply with another instance declaration. For instance: class C a b; instance C a (); instance C () a; instance C () (); As you can see, the first two instances partially overlap, but the third one disambiguates. I think it would be nice if GHC -fallow-overlapping-instances allowed this. This would take some of the pain out of overlapping instance resolution... -- Ashley Yakeley, Seattle WA

2 1

RE: infix type constructors
by Okasaki, C. DR EECS 16 May '02

16 May '02

Simon wrote: > I'm slowly getting around to this. Design questions: > > (A) I think it would be a good compromise to declare that operators > like "+" are type *constructors* not type *variables*. So > S+T > would be a type. That's slightly inconsistent with value variables, > but it's jolly useful. So only alphabetic things would be type > variables. > It's very clunky having to write > S :+: T As a design principle, I would recommend keeping type constructors and value constructors as similar as possible. So I would require infix type constructors to begin with a :, just like value constructors. Yes, it's clunky, but no more clunky than for value constructors. > (B) One wants to declare fixities for type constructors, and that > gets them mixed up with their value counterparts. My suggestion: > disamiguate with a compulsory 'type' keyword > infix 6 type + > infixl 9 type * > > Or should it be 'data'? Or should it depend how + and * are declared? My preference here would be for an infix declaration for a given name to apply to both type and value constructors. So, if you have a type constructor :- and a value constructor :-, they will have exactly the same precedence and associativity. I think it would be far too confusing for a type a :-: b :+: c to parse differently than an expression x :-: y :+: z > (C) The other place they can get mixed up is in import and export > lists. I can think of several solutions > > (i) module Foo( + ) where ... > means export the type constructor (+); currently illegal in H98 > module Foo( (+) ) where ... > means export the variable (+). > > This seems a bit of a hack. > > (ii) Use the 'type' keyword, rather like 'module': > module Foo( type + ) where > data a+b = A a | B b > or > module Foo( type +(A,B) ) where > data a+b = A a | B b > > [I think 'type' is better than 'data' because we want to hide the > distinction in an export list.... or do you think we should use the > same keyword as the one in the defn?] > > Similarly on import lists. If you keep the : prefix for infix type constructors, then this issue doesn't arise, just as it doesn't for alphabetic type and value constructors. > (D) I suppose one might want infix notation for type variables too: > > data T a = T (Int `a` Int) > > but maybe that's going too far? I don't have a strong feeling either way about this one. -- Chris

1 0

?? Redirecting standar error output ...
by heron_carvalho＠bol.com.br 16 May '02

16 May '02

Hello, How can I redirect standard error output to a file in GHC 5.02 ?? Something like -odump flag in GHC 4.08 ... Best Regards, Heron de Carvalho __________________________________________________________________________ Quer ter seu próprio endereço na Internet? Garanta já o seu e ainda ganhe cinco e-mails personalizados. DomíniosBOL - http://dominios.bol.com.br

1 0

RE: infix type constructors
by Simon Peyton-Jones 16 May '02

16 May '02

Chris I'm slowly getting around to this. Design questions: (A) I think it would be a good compromise to declare that operators like "+" are type *constructors* not type *variables*. So S+T would be a type. That's slightly inconsistent with value variables, but it's jolly useful. So only alphabetic things would be type variables. It's very clunky having to write S :+: T (B) One wants to declare fixities for type constructors, and that gets them mixed up with their value counterparts. My suggestion: disamiguate with a compulsory 'type' keyword infix 6 type + infixl 9 type * Or should it be 'data'? Or should it depend how + and * are declared? (C) The other place they can get mixed up is in import and export lists. I can think of several solutions (i) module Foo( + ) where ... means export the type constructor (+); currently illegal in H98 module Foo( (+) ) where ... means export the variable (+). This seems a bit of a hack. (ii) Use the 'type' keyword, rather like 'module': module Foo( type + ) where data a+b = A a | B b or module Foo( type +(A,B) ) where data a+b = A a | B b [I think 'type' is better than 'data' because we want to hide the distinction in an export list.... or do you think we should use the same keyword as the one in the defn?] Similarly on import lists. (D) I suppose one might want infix notation for type variables too: data T a = T (Int `a` Int) but maybe that's going too far? Simon | -----Original Message----- | From: Okasaki, C. DR EECS [mailto:Christopher.Okasaki@usma.edu] | Sent: 03 May 2002 14:09 | To: 'glasgow-haskell-users(a)haskell.org' | Subject: infix type constructors | | | I'm not sure how long this has been implemented in GHC, | but I just noticed that infix type constructors are allowed, | as in | | data a :- b = ... | | The syntactic asymmetry between type constructors and | data contructors has bothered me for a while, so this | is a welcome change! However, this syntax seems to | be supported for "data" and "newtype" declarations, | but not for "type" declarations. For example, | | type a :- b = ... | | does not seem to be allowed. Is there a reason for this? | Or was it just an oversight? | | -- Chris | _______________________________________________ | Glasgow-haskell-users mailing list | Glasgow-haskell-users(a)haskell.org | http://www.haskell.org/mailman/listinfo/glasgow-| haskell-users |

1 0

RE: syntax of RULES pragmas?
by Simon Peyton-Jones 16 May '02

16 May '02

You need -fglasgow-exts. (Should ignore a pragma without -fglasgow-exts, and does so now, but 5.03 gave the bad message you found.) The manual is wrong; spaces between the variables is right. Simon | -----Original Message----- | From: Janis Voigtlaender [mailto:voigt@orchid.inf.tu-dresden.de] | Sent: 16 May 2002 14:49 | To: glasgow-haskell-users(a)haskell.org | Subject: syntax of RULES pragmas? | | | Hi, | | I was trying to play with GHC 5.02's RULES pragmas, but | failed due to syntax problems. | | When trying: | | {-# RULES "map/map" forall f g xs. map f (map g xs) = map | (f.g) xs #-} | | main = print (map id (map id "Hello")) | | I get: | | ghc5 test.hs -O | test.hs:1: Variable not in scope: `forall' | | test.hs:1: Variable not in scope: `f' | | test.hs:1: Variable not in scope: `g' | | test.hs:1: Variable not in scope: `xs' | | test.hs:1: Variable not in scope: `f' | | test.hs:1: Variable not in scope: `g' | | test.hs:1: Variable not in scope: `xs' | | test.hs:1: Variable not in scope: `f' | | test.hs:1: Variable not in scope: `g' | | test.hs:1: Variable not in scope: `xs' | Exit 1 | | | With: | | {-# RULES "map/map" forall f g xs. | map f (map g xs) = map (f.g) xs #-} | | main = print (map id (map id "Hello")) | | I get: | | test.hs:2: parse error (possibly incorrect indentation) | Exit 1 | | | In the user's doc on http://www.haskell.org/ghc/ I also saw | the syntax: | | {-# RULES "map/map" forall f,g,xs. map f (map g xs) = map | (f.g) xs #-} | | main = print (map id (map id "Hello")) | | which fails with: | | test.hs:1: parse error on input `,' | Exit 1 | | | So how exactly do I have to specify a rewrite rule? Any hints | appreciated. | | Thanks, Janis. | | | -- | Janis Voigtlaender | http://wwwtcs.inf.tu-dresden.de/~voigt/ | mailto:voigt@tcs.inf.tu-dresden.de | _______________________________________________ | Glasgow-haskell-users mailing list | Glasgow-haskell-users(a)haskell.org | http://www.haskell.org/mailman/listinfo/glasgow-| haskell-users |

1 0

syntax of RULES pragmas?
by Janis Voigtlaender 16 May '02

16 May '02

Hi, I was trying to play with GHC 5.02's RULES pragmas, but failed due to syntax problems. When trying: {-# RULES "map/map" forall f g xs. map f (map g xs) = map (f.g) xs #-} main = print (map id (map id "Hello")) I get: ghc5 test.hs -O test.hs:1: Variable not in scope: `forall' test.hs:1: Variable not in scope: `f' test.hs:1: Variable not in scope: `g' test.hs:1: Variable not in scope: `xs' test.hs:1: Variable not in scope: `f' test.hs:1: Variable not in scope: `g' test.hs:1: Variable not in scope: `xs' test.hs:1: Variable not in scope: `f' test.hs:1: Variable not in scope: `g' test.hs:1: Variable not in scope: `xs' Exit 1 With: {-# RULES "map/map" forall f g xs. map f (map g xs) = map (f.g) xs #-} main = print (map id (map id "Hello")) I get: test.hs:2: parse error (possibly incorrect indentation) Exit 1 In the user's doc on http://www.haskell.org/ghc/ I also saw the syntax: {-# RULES "map/map" forall f,g,xs. map f (map g xs) = map (f.g) xs #-} main = print (map id (map id "Hello")) which fails with: test.hs:1: parse error on input `,' Exit 1 So how exactly do I have to specify a rewrite rule? Any hints appreciated. Thanks, Janis. -- Janis Voigtlaender http://wwwtcs.inf.tu-dresden.de/~voigt/ mailto:voigt@tcs.inf.tu-dresden.de

1 0

Exceptions and IO
by Ashley Yakeley 16 May '02

16 May '02

My confusion surrounding exceptions in the IO monad comes from the fact that IO failures and "bottom" are not cleanly separated. I had always assumed the IO monad looked something like this: newtype IO a = IO (RealWorldState -> Either IOFailure (RealWorldState,a)) return a = IO (\r -> Right (r,a)) fail s = IO (\r -> Left (userFailure s)) This would make sense, I think, because it's so easy this way for Prelude.catch to catch all IOFailures but leave pure "bottom" exceptions alone, just as the report says. But in fact IO looks more like this: newtype IO a = IO (RealWorldState -> (RealWorldState,a)) return a = IO (\r -> (r,a)) fail s = IO (\r -> throw (userError s)) ...which means Prelude.catch has to separate out exceptions caused by "fail" from those caused by error, etc. and there's confusion between "bottom" and exceptions that happen entirely in IO. -- Ashley Yakeley, Seattle WA

2 1

?? Redirecting standar error output ...
by Heron 16 May '02

16 May '02

Hello, How can I redirect standard error output to a file in GHC 5.02 ?? Something like -odump flag in GHC 4.08 ... Best Regards, Heron de Carvalho __________________________________________________________________ Heron Carvalho MSc. ICQ#: 117117637 Current ICQ status: + More ways to contact me i See more about me: __________________________________________________________________

1 0

RE: Prelude.catch vs. Exception.catch
by Ashley Yakeley 16 May '02

16 May '02

At 2002-05-14 04:10, I wrote: >>what's the motivation for this change? > >Well I think it's more intuitive. FWIW, I'd like to retract this pending further investigation. I'm still not sure I fully understand how exceptions and failures work in the IO monad. -- Ashley Yakeley, Seattle WA

1 0