- Glasgow-haskell-users

Enabling TypeHoles by default
by Krzysztof Gogolewski 15 Jan '14

15 Jan '14

Hello, As discussed on ghc-devs, I propose to enable -XTypeHoles in GHC by default. Rationale: (1) This way holes are far easier to use; just entering "_" allows to check type of a subexpression, no need of adding "-XTypeHoles". (2) This affects error messages only; i.e. the set of programs that compile stays the same - Haskell 2010. The only exception is that if you use -fdefer-type-errors, then a program with a hole compiles, but this seems fine with philosophy of -fdefer-type-errors. If so: would you like it to be in 7.8 or wait a cycle? My preference is 7.8, two people (John and Richard) suggested 7.10. -KG

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Feature request: Vacuous/error constraint (related to 7.7 closed type families regression)
by Merijn Verstraaten 14 Jan '14

14 Jan '14

I was trying to fix one of my closed type families examples for the new syntax, and run into an unfortunate issue. Due to a kind error in my code one of my constraints was being silently discarded (see ticket: https://ghc.haskell.org/trac/ghc/ticket/8669) The main reason I resorted to this (in hindsight) ill-kinded hack is the lack of a vacuous (i.e. never holding) constraint to produce type errors. I would love to be able to explicitly force GHC into realising "this constraint can never hold", if I had the ability to pass along a String/Symbol to be reported to the user, that would be even better. Cheers, Merijn

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Problem with the object file name of the Main module.
by Christian Brolin 14 Jan '14

14 Jan '14

H. The documentation (version 7.6.3) section 7.7.2 Output files says that the source file name of the Main module does not have to be Main.hs making it "possible to have several Main modules in separate source files in the same directory". While this is true and promising it is seems to be unusable since GHC always compiles the main source file into an object file called Main.o.

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Mips64el Linux Build
by cherry 04 Jan '14

04 Jan '14

Hello, I am new to the list, though I have been using GHC for a long time. Just say Hello. I would like to share my experience about building GHC (7.7.20131112) on/for mips64el linux, just in case anybody is interested, as it seems not documented (on Wiki pages of Platforms, CrossCompiling, or Porting). Ignore this email if it is not interesting or too trivial. It bootstrapped by cross compiling on x86_64 linux, with some tweaking on mk/build.mk and the build process. In particular, I needed to specify export CPP="mips64el-unknown-linux-gnu-gcc -E" otherwise the configure script just found to use the native "gcc -E" and it failed in preprocessing rts C files. Also in mk/build.mk I forced GhcThreaded = NO GhcWithInterpreter = YES # so, it can use TH and build happy The make process failed when trying to use ghc-stage2, but at that point the stage 2 compiler is already built, that's enough. With this cross-compiled ghc-stage2, the native build on mips64el machine was done without much interleaving. inplace/lib/unlit was x86_64 binary, and needed to be rebuild. Also manually built alex and happy (and their dependencies). Then it just went through. So, the new way of porting seems just cross compiling followed by native compiling? Somewhat surprisingly, GHCi is also working (forced it to be built), with system's dynamic linker. Is there need to write/update a wiki page for this? Thanks, cherry

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memory ordering
by John Lato 01 Jan '14

01 Jan '14

Hello, I'm working on a lock-free algorithm that's meant to be used in a concurrent setting, and I've run into a possible issue. The crux of the matter is that a particular function needs to perform the following: > x <- MVector.read vec ix > position <- readIORef posRef and the algorithm is only safe if these two reads are not reordered (both the vector and IORef are written to by other threads). My concern is, according to standard Haskell semantics this should be safe, as IO sequencing should guarantee that the reads happen in-order. Of course this also relies upon the architecture's memory model, but x86 also guarantees that reads happen in order. However doubts remain; I do not have confidence that the code generator will handle this properly. In particular, LLVM may freely re-order loads of NotAtomic and Unordered values. The one hope I have is that ghc will preserve IO semantics through the entire pipeline. This seems like it would be necessary for proper handling of exceptions, for example. So, can anyone tell me if my worries are unfounded, or if there's any way to ensure the behavior I want? I could change the readIORef to an atomicModifyIORef, which should issue an mfence, but that seems a bit heavy-handed as just a read fence would be sufficient (although even that seems more than necessary). Thanks, John L.

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Decomposition of given equalities
by Gábor Lehel 31 Dec '13

31 Dec '13

Hello, The upcoming GHC 7.8 recently gave me this error: Could not deduce (i ~ i1) from the context (f1 i ~ f i1) Which is strange to me: shouldn't (f1 i ~ f i1) exactly imply (f1 ~ f, i ~ i1)? (Or with nicer variable names: (f a ~ g b) => (f ~ g, a ~ b)?) When I inquired about this in #haskell on IRC, a person going by the name xnyhps had this to say: > I've also noticed that, given type equality constraints are never decomposed. I'm quite curious why. and later: > It's especially weird because a given f a ~ g b can not be used to solve a wanted f a ~ g b, because the wanted constraint is decomposed before it can interact with the given constraint. I'm not quite so well versed in the workings of GHC's type checker as she or he is, but I don't understand why it's this way either. Is this a relic of https://ghc.haskell.org/trac/ghc/ticket/5591 and https://ghc.haskell.org/trac/ghc/ticket/7205? Is there a principled reason this shouldn't be true? Is it an intentional limitation of the constraint solver? Or is it just a bug? Thanks in advance, Gábor P.S. I got the error on this line: https://github.com/glaebhoerl/type-eq/blob/master/Type/Eq.hs#L181, possibly after having added kind annotations to `InnerEq` (which also gets a less general kind inferred than the one I expect). If it's important I can try to create a reduced test case.

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Excellent bikeshedding opportunity! Frontend syntax for pattern synonym types
by Dr. ERDI Gergo 25 Dec '13

25 Dec '13

Hi, As part of my work to add pattern synonyms to GHC (https://ghc.haskell.org/trac/ghc/ticket/5144) I initially planned to postpone implementing type signatures (https://ghc.haskell.org/trac/ghc/ticket/8584) However, since adding the feature in the first place requires Haddock support, some syntax will be needed for pattern synonym type signatures so that at least there's something to generate into the docs. The basic problem with pattern synonyms in this regard is that their type is fully described by the following five pieces of information: 1, Universially bound type variables 2, Existentially bound type variables 3, The (tau) type itself 4, Typeclass context required by the pattern synonym 5, Typeclass context provided by the pattern synonym To give you some parallels, functions are described by (1), (3) and (4), e.g. given the following definition: f = map fromIntegral (1) is {a, b} (3) is [a] -> [b] (4) is (Integral a, Num b) Data constructors are described by (1), (2), (3) and (5), e.g. given data T a where MkT :: (Num a, Eq b) => a -> (b, b) -> T a the type of `MkT` is described by (1) Universially bound type variables {a} (2) Existentially bound type variables {b} (3) Tau type a -> (b, b) -> T a (5) Typeclass context provided is (Num a, Eq b) Note that when using `MkT` in an expression, its type is simpler than that, since it simply becomes forall a b. (Num a, Eq b) => a -> (b, b) -> T a which has exactly the same shape as the previous example which had (1), (3) and (4) specified. However, the context has different semantics (and the distinction between (1) and (2) becomes important) when pattern matching on `MkT`. For pattern synonyms, all five axes are present and orthogonal, and all five is something that a user should care about. For example, given the following code: {-# LANGUAGE PatternSynonyms, GADTs, ViewPatterns #-} data T a where MkT :: (Num a, Eq b) => a -> b -> T a f :: (Show a) => a -> Bool f = ... pattern P x <- MkT (f -> True) x to fully describe the type of pattern synonym P, we have: (1) Universially quantified type variables {a} (2) Existentially quantified type variables {b} (3) Tau type b -> T a (4) Required context (Show a) (5) Provided context (Num a, Eq b) So, what I'm looking for, is ideas on what syntax to use to represent these five pieces of information. Note that (1) and (2) can be made implicit, just like for constructor types, simply by noting that type variables that don't occur in the result type are existentially bound. So the tricky part is maintaining the distinction between (4) and (5). The current implementation displays the following if you ask for `:info T`: > :info P pattern P :: b -> T t requires (Show t) provides (Num t, Eq b) but I don't think we would want to use that for syntax that is actually enterred by the user into type signatures (if nothing else, then simply because why would we want to use two extra keywords 'requires' and 'provides'). The one idea I've had so far is to separate (4), (3) and (5) with two double arrows: pattern P :: (Show t) => b -> T t => (Num t, Eq b) As an added extra problem, there are also unidirectional and bidirectional pattern synonyms: unidirectional ones are usable only as patterns, whereas bidirectional ones can also be used as expressions. For example: pattern Single x = [x] pattern Second x <- (_:x:_) in this example, `Single` is bidirectional and `Second` is unidirectional. As you can see, this is indicated by syntax in the definition (`=` vs `<-`). However, I'd like to show this in the type as well, since you'd need to be able to see if you can use a given pattern synonym as a "constructor", not just a "destructor", by just looking at its Haddock-generated docs. What do you think? Bye, Gergo -- .--= ULLA! =-----------------. \ http://gergo.erdi.hu \ `---= gergo(a)erdi.hu =-------' Either the chocolate in my pocket has melted, or this is something altogether more sinister.

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ghc src snapshots
by Jens Petersen 24 Dec '13

24 Dec '13

Are recent source tarball snapshots available somewhere? (I mean more recent than http://darcs.haskell.org/ghcBuilder/uploads/ :) Anyway since I haven't seen any recently, I finally rolled one today and uploaded ghc-7.7.20131005-src.tar.bz2 (11MB) to: http://petersen.fedorapeople.org/ghc/ It seems to build ok for me. :) (I didn't go to the trouble of generating a testsuite tarball too.) Thanks, Jens

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Fwd: Decomposition of given equalities
by Thijs Alkemade 19 Dec '13

19 Dec '13

[Resending to the list, I used the wrong address. Sorry for the duplicate copies.] On 19 dec. 2013, at 05:30, Richard Eisenberg <eir(a)cis.upenn.edu> wrote: > I'd say GHC has it right in this case. > > (f a ~ g b) exactly implies (f ~ g) and (a ~ b) if and only if the kinds match up. If, say, (f :: k1 -> *), (g :: k2 -> *), (a :: k1), and (b :: k2), then (f ~ g) and (a ~ b) are ill-kinded. In Gabor's initial problem, we have (with all type, kind, and coercion variables made explicit) But even when it can know the kinds match up it doesn’t decompose it. For example: — {-# LANGUAGE TypeFamilies #-} foo :: (f a ~ g b) => f a -> g b foo = id — Ends up with 2 errors (7.6.3): Could not deduce (a ~ b) from the context (f a ~ g b) Could not deduce (f ~ g) from the context (f a ~ g b) So the wanted constraint f a ~ g b was decomposed, meaning the kind of a and b, and f and g were already found to be equal. Even with an explicit kind signature on every variable it doesn't work: foo :: (f a ~ g b) => (f :: * -> *) (a :: *) -> (g :: * -> *) (b :: *) I can’t think of an example where f a ~ g b holds, but the kinds of a and b are different. Thijs (“xnyhps”)

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Better GHC builds for Windows users
by Austin Seipp 09 Dec '13

09 Dec '13

Hello all, After fiddling around this weekend I've written a quick, comprehensive guide to a Better Build Environment for windows users. It pretty much "Just works" and I think should make it easy to get started with new machines: https://ghc.haskell.org/trac/ghc/wiki/Building/Preparation/Windows/MSYS2 Many thanks to Kyra for pointing this out to us on the GHC developer list. It is a much nicer to use mingw/msys environment with a lot of bugs fixed, and it's easy to install. Following the instructions should mostly be copy-paste. In particular, this resolves some weird issues with `validate` and traditional `make`, including a horrible deadlocking bug with make (and also 'tee', it seems.) So windows users can build in parallel again and everything works as expected. It also gives a simple and complete guide to build for x86_64 users (we didn't really mention it previously.) Please shout if you have problems. I've tested this with both an x86 and x86_64 build. -- Regards, Austin Seipp, Haskell Consultant Well-Typed LLP, http://www.well-typed.com/

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