November 2016 - Glasgow-haskell-users

GHC rewrite rules for class operations & laws
by Conal Elliott 04 Jan '17

04 Jan '17

Is it possible to apply GHC rewrite rules to class methods? From what I’ve read and seen, class methods get eliminated early by automatically-generated rules. Is there really no way to postpone such inlining until a later simplifier stage? The GHC Users Guide docs say no <https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/glasgow_ext…>, and suggests instead giving a duplicate vocabulary with somewhat awkward names for class methods. I’ve not seen this practice in libraries. I gather that we cannot therefore use class laws as optimizations in the form of rewrite rules, which seems a terrible loss. In Control.Category and Control.Arrow, I see rules for class laws but also header comments saying “The RULES for the methods of class Arrow may never fire e.g. compose/arr; see Trac #10528”. I’d appreciate a reality check about my conclusions as well as any strategies for using class laws in optimization. Thanks, -- Conal

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Allow extra commas in module declarations or lists?
by Michael Burge 17 Dec '16

17 Dec '16

Some languages like Perl allow you to include additional commas in your lists, so that you can freely reorder them without worrying about adding or removing commas from the last or first item: my @array = ( 1, 2, 3, ); Perl even allows multiple redundant commas everywhere after the first element, which is less useful but has come up on occasion: my @array = (1,,,2,,,,,,,,,3,,,,,); I propose allowing an optional single extra comma at the end in module declarations, record declarations, record constructors, and list constructors: module Example ( module Example, SomeConstructor(..), ) where data SomeConstructor = SomeConstructor { foo :: Int, bar :: Int, } baz :: SomeConstructor -> SomeConstructor baz x = x { foo = 5, bar = 10, } qux :: [ Int ] qux = [ 1, 2, 3, ] What do you think?

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GHC API: memory usage of loaded modules
by Evan Laforge 15 Dec '16

15 Dec '16

I have a program that uses the GHC API to provide a REPL. It winds up taking up 200mb in RAM, as measured by GHC.Stats.currentBytesUsed, but without the GHC API it's 5mb. If I turn on verbose, I can see that GHC is loading 255 modules, all loaded binary ("skipping M ( M.hs, M.hs.o )") except the toplevel, and the memory use is zooming up as it loads them. I expect some memory usage from loading modules, but 195mb seems like a lot. If I do a 'du' on the entire obj directory (which has 401 *.hs.o files... the REPL doesn't expose everything), it's only 76mb on disk. How do loaded modules wind up consuming space, and is there any way to use less space? The thing is, all those loaded modules are part of the application itself, so presumably they've already been linked into the binary and loaded into memory. The ideal would be that I could somehow reuse that. I imagine that I could by writing my own haskell interpreter and making a big symbol table of all the callable functions, but I'd rather not write my own interpreter if I can use an existing one!

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[ANNOUNCE] GHC 8.0.2 release candidate 1
by Ben Gamari 29 Nov '16

29 Nov '16

Hello everyone, The GHC team is happy to (finally!) announce the first candiate of the 8.0.2 release of the Glasgow Haskell Compiler. Source and binary distributions are available at http://downloads.haskell.org/~ghc/8.0.2-rc1/ This is the first of what will hopefully be only two release candidates leading up the final 8.0.2 release. This release will fix a number bugs found in 8.0.1 including, * Interface file build determinism (#4012). * Compatibility with macOS Sierra and GCC compilers which compile position-independent executables by default * Runtime linker fixes on Windows (see #12797) * A compiler bug which resulted in undefined reference errors while compiling some packages (see #12076) * Compatability with systems which use the gold linker * A number of memory consistency bugs in the runtime system * A number of efficiency issues in the threaded runtime which manifest on larger core counts and large numbers of bound threads. * A typechecker bug which caused some programs using -XDefaultSignatures to be incorrectly accepted. * More than two-hundred other bugs. See Trac [1] for a complete listing. Unfortunately there is one known bug (#12757) which can result in runtime crashes which is still lurking in -rc1. This will be fixed in -rc2, which will be released in about a week. As always, let us know if you encounter trouble. Thanks to everyone who has contributed so far! Happy testing, - Ben [1] https://ghc.haskell.org/trac/ghc/query?status=closed&milestone=8.0.2&col=id…

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