May 2002 - Glasgow-haskell-users

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

efficiency of UArray
by Hal Daume III 15 May '02

15 May '02

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

1 0

RE: using error x as a placeholder
by Simon Marlow 15 May '02

15 May '02

[ moved to glasgow-haskell-users(a)haskell.org ] > The timing results were: > > mkArray1: 11.42u 0.79s 0:13.47 90.6% > mkArray2: 24.55u 2.31s 0:30.12 89.1% > > Which is actually *slower*. Any ideas why? (These were > compiled with ghc > 5.02.3 -O2 -fvia-c -fall-strict) What are the results without -fall-strict? (-fall-strict is an old experimental flag and almost certainly doesn't do anything reasonable. We've de-documented it in the HEAD). Cheers, Simon

2 1

RE: Haddock installing problems
by Simon Marlow 15 May '02

15 May '02

[ moved to glasgow-haskell-users(a)haskell.org, which is slightly more appropriate than haskell ] > Looking the output produced by ./configure, I noticed that: > > checking for ghc... /PROGS/GHC5023/BIN/ghc > checking version of ghc... /PROGS/GHC5023/BIN/ghc: not found > > I wonder why haddock thinks my ghc dir is > "/PROGS/GHC5023/BIN/ghc" when it > should be "/progs/ghc5023/bin/ghc". Perhaps that's why it > can't find perl: > ghc itself was not found. Pathnames aren't case-sensitive on Win32 are they? So why should that make a difference? What is your cygwin mount setup? What is mounted on /? Cheers, Simon

1 0

RE: Replacing the Prelude
by Simon Peyton-Jones 14 May '02

14 May '02

Ashley writes | > I was hoping to do something similar for 'do' notation by redefining | > (>>), (>>=) etc., but unfortunately GHC is quite insistent | that 'do' notation quite specifically refers to GHC.Base.Monad Dylan replies | I'm surprised that ghc uses the fromInteger and fromRational | that are in scope; I thought there was general agreement that | it should use the Prelude.from{Integer,Rational} in scope. Ashley is referring to a GHC extension. Normally, GHC uses Prelude.fromInteger etc, regardless of what is in scope. But if you say -fno-implicit-prelude, GHC will instead use whatver fromInteger is in scope. (This is documented in the manual.) Ashley's question, as I understand is whether something similar could be done for monads. Answer: I think so, without much bother. I'm beavering away on a Haskell workshop paper at the moment, but ping me in a fortnight to do it. Simon

1 0

Replacing the Prelude
by Ashley Yakeley 14 May '02

14 May '02

I have recently been experimenting writing code that replaces large chunks of the Prelude, compiling with -fno-implicit-prelude. I notice that I can happily redefine numeric literals simply by creating functions called 'fromInteger' and 'fromRational': GHC will use whatever is in scope for those names. I was hoping to do something similar for 'do' notation by redefining (>>), (>>=) etc., but unfortunately GHC is quite insistent that 'do' notation quite specifically refers to GHC.Base.Monad (i.e. Prelude.Monad, as the Report seems to require). I don't suppose there's any way of fooling it, is there? I was rather hoping 'do' notation would work like a macro in rewriting its block, and not worry about types at all. I accept that this might be a slightly bizarre request. There are a number of things I don't like about the way the Prelude.Monad class and 'do' notation are set up, and it would be nice to be able to experiment with alternatives. -- Ashley Yakeley, Seattle WA

2 1

RE: Prelude.catch vs. Exception.catch
by Ashley Yakeley 14 May '02

14 May '02

At 2002-05-14 03:57, Simon Marlow wrote: >It turns out that the compiler "bug" is really just the compiler being a >bit loose with the IO monad - it freely translates the original >definition of evaluate using 'seq' into the slightly less strict version >by pushing the 'seq' through the state lambda of the IO monad (this only >happens for the IO monad, and strictly speaking it's a deviation from >the semantics but it has important performance benefits for IO code). Under what circumstance would the compiler do this to my code? I really need to know about any deviation from the semantics. >so your catch can be defined in terms of the current catch like so: > > catch' a h = a `seq` (catch a h) Oh yeah. >what's the motivation for this change? Well I think it's more intuitive. The documentation (sec. 5.12.3) says this: The simplest version is catch. It takes a single argument, runs it, and if an exception is raised the "handler" is executed passing the value of the exception. Otherwise, the result is returned as normal. ...leading to the question, how do you 'run' bottom? But since I can do what I want with your "catch'" function, it's not really that important. -- Ashley Yakeley, Seattle WA

1 0