- Haskell-Cafe - Haskell.org

Composing Enumeratees in enumerator
by Michael Craig 02 Jan '12

02 Jan '12

I've been looking for a way to compose enumeratees in the enumerator package, but I've come up with nothing so far. I want this function (=$=) :: Monad m => Enumeratee a0 a1 m b -> Enumeratee a1 a2 m b -> Enumeratee a0 a2 m b I'm building a modular library on top of enumerator that facilitates reading time series data from a DB, applying any number of transformations to it, and then writing it back / doing something else with it. I'd like to be able to write simple transformations (enumeratees) and compose them without binding them to either a db reader (enumerator) or db writer (iteratee). I've been looking at the iterIO package as a possible alternative, because it seems to allow easy composition of Inums (enumeratees). I'm a little skittish of it because it seems unpopular next to enumerator. Thoughts on these issues? Cheers, Mike S Craig

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Installing glib
by Victor Miller 02 Jan '12

02 Jan '12

I have the Haskell Platform (version 7.0.4 of ghc) installed on my macbook (OS X 10.7 Lion) and I'd like to install glib. However, when I try cabal install glib, I get: cabal install glib Resolving dependencies... [1 of 2] Compiling SetupWrapper ( /tmp/glib-0.12.25777/glib-0.12.2/SetupWrapper.hs, /tmp/glib-0.12.25777/glib-0.12.2/dist/setup/SetupWrapper.o ) [2 of 2] Compiling Main ( /tmp/glib-0.12.25777/glib-0.12.2/Setup.hs, /tmp/glib-0.12.25777/glib-0.12.2/dist/setup/Main.o ) Linking /tmp/glib-0.12.25777/glib-0.12.2/dist/setup/setup ... ld: warning: could not create compact unwind for _ffi_call_unix64: does not use RBP or RSP based frame [1 of 2] Compiling Gtk2HsSetup ( Gtk2HsSetup.hs, dist/setup-wrapper/Gtk2HsSetup.o ) [2 of 2] Compiling Main ( SetupMain.hs, dist/setup-wrapper/Main.o ) Linking dist/setup-wrapper/setup ... ld: warning: could not create compact unwind for _ffi_call_unix64: does not use RBP or RSP based frame Cannot find gtk2hsC2hs Please install `gtk2hs-buildtools` first and check that the install directory is in your PATH (e.g. HOME/.cabal/bin). cabal: Error: some packages failed to install: glib-0.12.2 failed during the configure step. The exception was: ExitFailure 1 ------- I've tried cabal install gtk2hs-buildtools, and appears to complete (it does have two compilation warnings), but that doesn't help. Any suggestions? Victor Miller Here's the result cabal install gtk2hs-buildtools: bash-3.2$ sudo cabal install gtk2hs-buildtools Resolving dependencies... Configuring gtk2hs-buildtools-0.12.1... Preprocessing executables for gtk2hs-buildtools-0.12.1... Building gtk2hs-buildtools-0.12.1... [1 of 2] Compiling Paths_gtk2hs_buildtools ( dist/build/autogen/Paths_gtk2hs_buildtools.hs, dist/build/gtk2hsTypeGen/gtk2hsTypeGen-tmp/Paths_gtk2hs_buildtools.o ) [2 of 2] Compiling Main ( hierarchyGen/TypeGen.hs, dist/build/gtk2hsTypeGen/gtk2hsTypeGen-tmp/Main.o ) Linking dist/build/gtk2hsTypeGen/gtk2hsTypeGen ... ld: warning: could not create compact unwind for _ffi_call_unix64: does not use RBP or RSP based frame [1 of 2] Compiling Paths_gtk2hs_buildtools ( dist/build/autogen/Paths_gtk2hs_buildtools.hs, dist/build/gtk2hsHookGenerator/gtk2hsHookGenerator-tmp/Paths_gtk2hs_buildtools.o ) [2 of 2] Compiling Main ( callbackGen/HookGenerator.hs, dist/build/gtk2hsHookGenerator/gtk2hsHookGenerator-tmp/Main.o ) Linking dist/build/gtk2hsHookGenerator/gtk2hsHookGenerator ... ld: warning: could not create compact unwind for _ffi_call_unix64: does not use RBP or RSP based frame [ 1 of 41] Compiling DLists ( c2hs/base/general/DLists.hs, dist/build/gtk2hsC2hs/gtk2hsC2hs-tmp/DLists.o ) dist/build/gtk2hsC2hs/gtk2hsC2hs-tmp/CLexer.hs:1:12: Warning: -fglasgow-exts is deprecated: Use individual extensions instead [ 2 of 41] Compiling BaseVersion ( c2hs/base/admin/BaseVersion.hs, dist/build/gtk2hsC2hs/gtk2hsC2hs-tmp/BaseVersion.o ) [ 3 of 41] Compiling Switches ( c2hs/state/Switches.hs, dist/build/gtk2hsC2hs/gtk2hsC2hs-tmp/Switches.o ) [ 4 of 41] Compiling Map ( c2hs/base/general/Map.hs, dist/build/gtk2hsC2hs/gtk2hsC2hs-tmp/Map.o ) [ 5 of 41] Compiling FastMutInt ( c2hs/base/general/FastMutInt.hs, dist/build/gtk2hsC2hs/gtk2hsC2hs-tmp/FastMutInt.o ) [ 6 of 41] Compiling Config ( c2hs/base/admin/Config.hs, dist/build/gtk2hsC2hs/gtk2hsC2hs-tmp/Config.o ) [ 7 of 41] Compiling C2HSConfig ( c2hs/toplevel/C2HSConfig.hs, dist/build/gtk2hsC2hs/gtk2hsC2hs-tmp/C2HSConfig.o ) [ 8 of 41] Compiling CInfo ( c2hs/gen/CInfo.hs, dist/build/gtk2hsC2hs/gtk2hsC2hs-tmp/CInfo.o ) [ 9 of 41] Compiling Version ( c2hs/toplevel/Version.hs, dist/build/gtk2hsC2hs/gtk2hsC2hs-tmp/Version.o ) [10 of 41] Compiling Binary ( c2hs/base/general/Binary.hs, dist/build/gtk2hsC2hs/gtk2hsC2hs-tmp/Binary.o ) [11 of 41] Compiling Position ( c2hs/base/general/Position.hs, dist/build/gtk2hsC2hs/gtk2hsC2hs-tmp/Position.o ) [12 of 41] Compiling UNames ( c2hs/base/general/UNames.hs, dist/build/gtk2hsC2hs/gtk2hsC2hs-tmp/UNames.o ) [13 of 41] Compiling Errors ( c2hs/base/errors/Errors.hs, dist/build/gtk2hsC2hs/gtk2hsC2hs-tmp/Errors.o ) [14 of 41] Compiling Attributes ( c2hs/base/syms/Attributes.hs, dist/build/gtk2hsC2hs/gtk2hsC2hs-tmp/Attributes.o ) [15 of 41] Compiling Idents ( c2hs/base/syms/Idents.hs, dist/build/gtk2hsC2hs/gtk2hsC2hs-tmp/Idents.o ) [16 of 41] Compiling CTokens ( c2hs/c/CTokens.hs, dist/build/gtk2hsC2hs/gtk2hsC2hs-tmp/CTokens.o ) [17 of 41] Compiling CAST ( c2hs/c/CAST.hs, dist/build/gtk2hsC2hs/gtk2hsC2hs-tmp/CAST.o ) [18 of 41] Compiling CPretty ( c2hs/c/CPretty.hs, dist/build/gtk2hsC2hs/gtk2hsC2hs-tmp/CPretty.o ) [19 of 41] Compiling Lexers ( c2hs/base/syntax/Lexers.hs, dist/build/gtk2hsC2hs/gtk2hsC2hs-tmp/Lexers.o ) [20 of 41] Compiling StateTrans ( c2hs/base/state/StateTrans.hs, dist/build/gtk2hsC2hs/gtk2hsC2hs-tmp/StateTrans.o ) [21 of 41] Compiling StateBase ( c2hs/base/state/StateBase.hs, dist/build/gtk2hsC2hs/gtk2hsC2hs-tmp/StateBase.o ) [22 of 41] Compiling NameSpaces ( c2hs/base/syms/NameSpaces.hs, dist/build/gtk2hsC2hs/gtk2hsC2hs-tmp/NameSpaces.o ) [23 of 41] Compiling CAttrs ( c2hs/c/CAttrs.hs, dist/build/gtk2hsC2hs/gtk2hsC2hs-tmp/CAttrs.o ) [24 of 41] Compiling CBuiltin ( c2hs/c/CBuiltin.hs, dist/build/gtk2hsC2hs/gtk2hsC2hs-tmp/CBuiltin.o ) [25 of 41] Compiling CParserMonad ( c2hs/c/CParserMonad.hs, dist/build/gtk2hsC2hs/gtk2hsC2hs-tmp/CParserMonad.o ) [26 of 41] Compiling CLexer ( dist/build/gtk2hsC2hs/gtk2hsC2hs-tmp/CLexer.hs, dist/build/gtk2hsC2hs/gtk2hsC2hs-tmp/CLexer.o ) dist/build/gtk2hsC2hs/gtk2hsC2hs-tmp/CLexer.hs:454:17: Warning: Bindings containing unlifted types should use an outermost bang pattern: base = alexIndexInt32OffAddr alex_base s In the expression: let base = alexIndexInt32OffAddr alex_base s (I# (ord_c)) = ord c offset = (base +# ord_c) .... in case new_s of { -1# -> (new_acc, input) _ -> alex_scan_tkn user orig_input (len +# 1#) new_input new_s new_acc } In a case alternative: Just (c, new_input) -> let base = alexIndexInt32OffAddr alex_base s (I# (ord_c)) = ord c .... in case new_s of { -1# -> (new_acc, input) _ -> alex_scan_tkn user orig_input (len +# 1#) new_input new_s new_acc } In the second argument of `seq', namely `case alexGetChar input of { Nothing -> (new_acc, input) Just (c, new_input) -> let base = ... .... in case new_s of { -1# -> ... _ -> alex_scan_tkn user orig_input (len +# 1#) new_input new_s new_acc } }' dist/build/gtk2hsC2hs/gtk2hsC2hs-tmp/CLexer.hs:455:17: Warning: Bindings containing unlifted types should use an outermost bang pattern: (I# (ord_c)) = ord c In the expression: let base = alexIndexInt32OffAddr alex_base s (I# (ord_c)) = ord c offset = (base +# ord_c) .... in case new_s of { -1# -> (new_acc, input) _ -> alex_scan_tkn user orig_input (len +# 1#) new_input new_s new_acc } In a case alternative: Just (c, new_input) -> let base = alexIndexInt32OffAddr alex_base s (I# (ord_c)) = ord c .... in case new_s of { -1# -> (new_acc, input) _ -> alex_scan_tkn user orig_input (len +# 1#) new_input new_s new_acc } In the second argument of `seq', namely `case alexGetChar input of { Nothing -> (new_acc, input) Just (c, new_input) -> let base = ... .... in case new_s of { -1# -> ... _ -> alex_scan_tkn user orig_input (len +# 1#) new_input new_s new_acc } }' dist/build/gtk2hsC2hs/gtk2hsC2hs-tmp/CLexer.hs:456:17: Warning: Bindings containing unlifted types should use an outermost bang pattern: offset = (base +# ord_c) In the expression: let base = alexIndexInt32OffAddr alex_base s (I# (ord_c)) = ord c offset = (base +# ord_c) .... in case new_s of { -1# -> (new_acc, input) _ -> alex_scan_tkn user orig_input (len +# 1#) new_input new_s new_acc } In a case alternative: Just (c, new_input) -> let base = alexIndexInt32OffAddr alex_base s (I# (ord_c)) = ord c .... in case new_s of { -1# -> (new_acc, input) _ -> alex_scan_tkn user orig_input (len +# 1#) new_input new_s new_acc } In the second argument of `seq', namely `case alexGetChar input of { Nothing -> (new_acc, input) Just (c, new_input) -> let base = ... .... in case new_s of { -1# -> ... _ -> alex_scan_tkn user orig_input (len +# 1#) new_input new_s new_acc } }' dist/build/gtk2hsC2hs/gtk2hsC2hs-tmp/CLexer.hs:457:17: Warning: Bindings containing unlifted types should use an outermost bang pattern: check = alexIndexInt16OffAddr alex_check offset In the expression: let base = alexIndexInt32OffAddr alex_base s (I# (ord_c)) = ord c offset = (base +# ord_c) .... in case new_s of { -1# -> (new_acc, input) _ -> alex_scan_tkn user orig_input (len +# 1#) new_input new_s new_acc } In a case alternative: Just (c, new_input) -> let base = alexIndexInt32OffAddr alex_base s (I# (ord_c)) = ord c .... in case new_s of { -1# -> (new_acc, input) _ -> alex_scan_tkn user orig_input (len +# 1#) new_input new_s new_acc } In the second argument of `seq', namely `case alexGetChar input of { Nothing -> (new_acc, input) Just (c, new_input) -> let base = ... .... in case new_s of { -1# -> ... _ -> alex_scan_tkn user orig_input (len +# 1#) new_input new_s new_acc } }' dist/build/gtk2hsC2hs/gtk2hsC2hs-tmp/CLexer.hs:459:17: Warning: Bindings containing unlifted types should use an outermost bang pattern: new_s = if (offset >=# 0#) && (check ==# ord_c) then alexIndexInt16OffAddr alex_table offset else alexIndexInt16OffAddr alex_deflt s In the expression: let base = alexIndexInt32OffAddr alex_base s (I# (ord_c)) = ord c offset = (base +# ord_c) .... in case new_s of { -1# -> (new_acc, input) _ -> alex_scan_tkn user orig_input (len +# 1#) new_input new_s new_acc } In a case alternative: Just (c, new_input) -> let base = alexIndexInt32OffAddr alex_base s (I# (ord_c)) = ord c .... in case new_s of { -1# -> (new_acc, input) _ -> alex_scan_tkn user orig_input (len +# 1#) new_input new_s new_acc } In the second argument of `seq', namely `case alexGetChar input of { Nothing -> (new_acc, input) Just (c, new_input) -> let base = ... .... in case new_s of { -1# -> ... _ -> alex_scan_tkn user orig_input (len +# 1#) new_input new_s new_acc } }' [27 of 41] Compiling FNameOps ( c2hs/base/general/FNameOps.hs, dist/build/gtk2hsC2hs/gtk2hsC2hs-tmp/FNameOps.o ) [28 of 41] Compiling FileOps ( c2hs/base/general/FileOps.hs, dist/build/gtk2hsC2hs/gtk2hsC2hs-tmp/FileOps.o ) [29 of 41] Compiling CIO ( c2hs/base/state/CIO.hs, dist/build/gtk2hsC2hs/gtk2hsC2hs-tmp/CIO.o ) [30 of 41] Compiling State ( c2hs/base/state/State.hs, dist/build/gtk2hsC2hs/gtk2hsC2hs-tmp/State.o ) [31 of 41] Compiling C2HSState ( c2hs/state/C2HSState.hs, dist/build/gtk2hsC2hs/gtk2hsC2hs-tmp/C2HSState.o ) [32 of 41] Compiling CHSLexer ( c2hs/chs/CHSLexer.hs, dist/build/gtk2hsC2hs/gtk2hsC2hs-tmp/CHSLexer.o ) [33 of 41] Compiling CHS ( c2hs/chs/CHS.hs, dist/build/gtk2hsC2hs/gtk2hsC2hs-tmp/CHS.o ) [34 of 41] Compiling GenHeader ( c2hs/gen/GenHeader.hs, dist/build/gtk2hsC2hs/gtk2hsC2hs-tmp/GenHeader.o ) [35 of 41] Compiling CTrav ( c2hs/c/CTrav.hs, dist/build/gtk2hsC2hs/gtk2hsC2hs-tmp/CTrav.o ) [36 of 41] Compiling CNames ( c2hs/c/CNames.hs, dist/build/gtk2hsC2hs/gtk2hsC2hs-tmp/CNames.o ) [37 of 41] Compiling CParser ( dist/build/gtk2hsC2hs/gtk2hsC2hs-tmp/CParser.hs, dist/build/gtk2hsC2hs/gtk2hsC2hs-tmp/CParser.o ) [38 of 41] Compiling C ( c2hs/c/C.hs, dist/build/gtk2hsC2hs/gtk2hsC2hs-tmp/C.o ) [39 of 41] Compiling GBMonad ( c2hs/gen/GBMonad.hs, dist/build/gtk2hsC2hs/gtk2hsC2hs-tmp/GBMonad.o ) [40 of 41] Compiling GenBind ( c2hs/gen/GenBind.hs, dist/build/gtk2hsC2hs/gtk2hsC2hs-tmp/GenBind.o ) [41 of 41] Compiling Main ( c2hs/toplevel/Main.hs, dist/build/gtk2hsC2hs/gtk2hsC2hs-tmp/Main.o ) Linking dist/build/gtk2hsC2hs/gtk2hsC2hs ... ld: warning: could not create compact unwind for _ffi_call_unix64: does not use RBP or RSP based frame Warning: No documentation was generated as this package does not contain a library. Perhaps you want to use the --executables flag. Installing executable(s) in /Users/victorsmiller/Library/Haskell/ghc-7.0.4/lib/gtk2hs-buildtools-0.12.1/bin Updating documentation index /Users/victorsmiller/Library/Haskell/doc/index.html

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Thanks and a happy new year!
by Ertugrul Söylemez 01 Jan '12

01 Jan '12

Dear fellow Haskellers, as I have learned so much throughout the past four years of Haskell experience, I feel obligated to thank everyone for the nice support and the great ideas. Exploring and using Haskell has changed my way of thinking about programming and computers in a profound way and made me a lot more productive. A happy new year 2012 for Haskell hacking to everyone! Many greets, Ertugrul -- nightmare = unsafePerformIO (getWrongWife >>= sex) http://ertes.de/

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Twidge using hashtags
by Martin de Bruin 31 Dec '11

31 Dec '11

Hello! fairly new to twidge. Got it installed and have it setting status etc etc . I'm maintaing a local website supporting the eating of a seasonal pastry in a highly competitve fashion. A race of sorts. The contestant eating the most from january first to good friday wins. To keep the eating as transparent as possible one must report each pastry on the web. However, I feel this is not very 2012. I'm thinking about going about the new version allowing reporting to be done via twitter and picking it up with Twidge. To keep everything as much in the open as possible I'd like to have all contestants twittering the eating of a pastry mentioning the "official twitter account of the website" and throw in a hashtag like #report and then some comment. I'll then read the feed periodically via cron on the server and update the highscore database. Am I making sense? The longer I typed the above the more I started thinkng about just doing something in PHP... Any thoughts? Cheers Martin

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Plugins on ghc 7.2: GHC does not export defaultCallbacks
by Brian Victor 31 Dec '11

31 Dec '11

Hi all, As a getting-my-feet-wet project I was starting to look into using plugins-auto with the yesod devel server, but I was quickly stymied because the plugins package isn't building on GHC 7.2. The error I get locally is the same as the one reported by hackage[1]. In short, defaultCallbacks seems to be gone from 7.2. It doesn't look like that function has simply been moved. So can someone give me an idea about how to proceed? Thanks! [1] http://hackage.haskell.org/packages/archive/plugins/1.5.1.4/logs/failure/gh… -- Brian

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Copy .cabal folder to diff machine/user
by Vagif Verdi 31 Dec '11

31 Dec '11

Is it possible to copy .cabal and .ghc folders to different machine/ user and develop same project over there ? Or is the only way to allow a team of developers to work on the same project is to force each one of them to install all necessary packages on their machines.

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Level of Win32 GUI support in the Haskell platform
by Steve Horne 30 Dec '11

30 Dec '11

I've been for functions like GetMessage, TranslateMessage and DispatchMessage in the Haskell Platform Win32 library - the usual message loop stuff - and not finding them. Hoogle says "no results found". Is this level of Win32 GUI coding supported? (other than by dealing with the FFI myself) Alternatively, should I be doing dialog-based coding and leaving Haskell to worry about message loops behind the scenes? Either way, is there an example I can use for reference? I have something along the lines of Petzolds Hello World in mind as the ideal (register a window class, handle the messages, paint some text), but anything that shows how to get a basic Win32 GUI app running in Haskell is fine.

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purity and the season of good will
by Chris Dornan 30 Dec '11

30 Dec '11

(Whether readers would consider the post [or indeed this post] an act of trollery was mooted and mentioned several times in the original post - my thoughts at the end. I am writing this because I would have expected somebody to have said this by now. If it has been said then my sincere apologies. Meta issues, flames, etc., to me and not the list of course.) Pure versus Impure I suspect introducing C into this discussion may not help clarify the fundamental issue. As anyone with experience of programming in Haskell appreciates, there is a big difference between (e.g.) these two functions for taking the sum of two integers: plus :: Int -> Int -> Int plusIO :: Int -> Int -> IO Int The choice is a profound one and will affect their construction as well as the contexts they can be used and so forth. Haskell supports both styles of programming and allows them to be safely and freely intermixed with the type system guaranteeing to keep them properly separated. The type system and static semantics are critical to the Haskell approach to IO and 'purity'. I can't see how it can be meaningfully pushed into the background in any discussion or comparative discussion of these issues. In Short If the programming context in question doesn't provide any equivalent formal separation in its type system of plus-like functions and plusIO-like computations, that programmers use in practice to guide the construction of their programs, then how can the said programming context be considered equivalent to Haskell's treatment of pure functions and effects-generating computations. You just need to consider your own Haskell programming activity and how the type system keeps calculations and I/O actions separate to see this point. I doubt if any extra theory (or theology!) will add much insight. That T word Again (or not) Let's keep this forward-looking. With the email piling in as I write this, please consider whether this continuation of this discussion is likely to be helpful to the general readers of this list or whether it will confuse or discourage them, before adding to it. A Happy New Year to you, Chris From: haskell-cafe-bounces(a)haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of Steve Horne Sent: 28 December 2011 17:40 To: Haskell Cafe Mailing List Subject: [Haskell-cafe] On the purity of Haskell This is just my view on whether Haskell is pure, being offered up for criticism. I haven't seen this view explicitly articulated anywhere before, but it does seem to be implicit in a lot of explanations - in particular the description of Monads in SBCs "Tackling the Awkward Squad". I'm entirely focused on the IO monad here, but aware that it's just one concrete case of an abstraction. Warning - it may look like trolling at various points. Please keep going to the end before making a judgement. To make the context explicit, there are two apparently conflicting viewpoints on Haskell... 1. The whole point of the IO monad is to support programming with side-effecting actions - ie impurity. 2. The IO monad is just a monad - a generic type (IO actions), a couple of operators (primarily return and bind) and some rules - within a pure functional language. You can't create impurity by taking a subset of a pure language. My view is that both of these are correct, each from a particular point of view. Furthermore, by essentially the same arguments, C is also both an impure language and a pure one. See what I mean about the trolling thing? I'm actually quite serious about this, though - and by the end I think Haskell advocates will generally approve. First assertion... Haskell is a pure functional language, but only from the compile-time point of view. The compiler manipulates and composes IO actions (among other things). The final resulting IO actions are finally swallowed by unsafePerformIO or returned from main. However, Haskell is an impure side-effecting language from the run-time point of view - when the composed actions are executed. Impurity doesn't magically spring from the ether - it results from the translation by the compiler of IO actions to executable code and the execution of that code. In this sense, IO actions are directly equivalent to the AST nodes in a C compiler. A C compiler can be written in a purely functional way - in principle it's just a pure function that accepts a string (source code) and returns another string (executable code). I'm fudging issues like separate compilation and #include, but all of these can be resolved in principle in a pure functional way. Everything a C compiler does at compile time is therefore, in principle, purely functional. In fact, in the implementation of Haskell compilers, IO actions almost certainly *are* ASTs. Obviously there's some interesting aspects to that such as all the partially evaluated and unevaluated functions. But even a partially evaluated function has a representation within a compiler that can be considered an AST node, and even AST nodes within a C compiler may represent partially evaluated functions. Even the return and bind operators are there within the C compiler in a sense, similar to the do notation in Haskell. Values are converted into actions. Actions are sequenced. Though the more primitive form isn't directly available to the programmer, it could easily be explicitly present within the compiler. What about variables? What about referential transparency? Well, to a compiler writer (and equally for this argument) an identifier is not the same thing as the variable it references. One way to model the situation is that for every function in a C program, all explicit parameters are implicitly within the IO monad. There is one implicit parameter too - a kind of IORef to the whole system memory. Identifiers have values which identify where the variable is within the big implicit IORef. So all the manipulation of identifiers and their reference-like values is purely functional. Actual handling of variables stored within the big implicit IORef is deferred until run-time. So once you accept that there's an implicit big IORef parameter to every function, by the usual definition of referential transparency, C is as transparent as Haskell. The compile-time result of each function is completely determined by its (implicit and explicit) parameters - it's just that that result is typically a way to look up the run-time result within the big IORef later. What's different about Haskell relative to C therefore... 1. The style of the "AST" is different. It still amounts to the same thing in this argument, but the fact that most AST nodes are simply partially-evaluated functions has significant practical consequences, especially with laziness mixed in too. There's a deep connection between the compile-time and run-time models (contrast C++ templates). 2. The IO monad is explicit in Haskell - side-effects are only permitted (even at run-time) where the programmer has explicitly opted to allow them. 3. IORefs are explicit in Haskell - instead of always having one you can have none, one or many. This is relevant to an alternative definition of referential transparency. Politicians aren't considered transparent when they bury the relevant in a mass of the irrelevant, and even pure functions can be considered to lack transparency in that sense. Haskell allows (and encourages) you to focus in on the relevant - to reference an IORef Bool or an IORef Int rather than dealing with an IORef Everything. That last sentence of the third point is my most recent eureka - not so long ago I posted a "Haskell is just using misleading definitions - it's no more transparent than C" rant, possibly on Stack Overflow. Wrong again :-( So - what do you think?

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Overloaded Quotes for Template Haskell
by Michael D. Adams 30 Dec '11

30 Dec '11

What would it take to get an "-XOverloadedQuotes" flag of the same sort as the "-XOverloadedStrings" flag? I.e. "[| ... |]" would have type "Quasi m => m Exp" instead of "Q Exp" and any splices in that quotation expect the contents of that splice to have type "m Exp". (Obviously, top level splices would still have type "Q Exp" otherwise the compiler wouldn't know how to evaluate the monad.) I ran into this problem when writing a Template Haskell program in which part of it operates in a "StateT S Q a" monad instead of the usual "Q a" monad. (The "S" type stores the state of a memoization table of code fragments already generated. Without it, the code would loop infinitely when processing certain recursive structures.) It is fairly easy to declare an instance of "Quasi" for "StateT S Q", so in order to keep the code clean, I'd like to use quotations with splices in them (i.e. [| ... $( ... ) ... |] ) for expressing the generated code. However, quotations and splices are tied to the "Q" monad which means that as it is now I have to manually write "LamE ... VarP ... VarE ... etc." instead of using the much nicer quotation syntax. Michael D. Adams mdmkolbe(a)gmail.com

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Type classes in Typing Haskell in Haskell
by Alexander Bau 30 Dec '11

30 Dec '11

Hi, recently I tried the Typing Haskell in Haskell library. But I was wondering why this program type checks: > -- plusMfun is standard '+': Num a => a -> a -> a > test = let Just classEnv = ( addCoreClasses <:> addNumClasses ) > initialEnv > e = Ap ( Ap (Var "+") (Lit $ LitStr "3")) (Lit $ LitStr "5") > impl = ("foo",[([],e)]) > in > putStrLn $ pretty $ runTI $ tiImpls classEnv [plusMfun] [impl] I was expecting some kind of typechecking error, because [Char] is no instance of Num. But I get this: > ([isIn1 cNum (TAp tList tChar)], > ["foo" :>: > Forall [] > ([] :=> > (TAp tList tChar))]) The predicate says that if [Char] would be in Num then the type of the expression would be [Char]. But actually [Char] isn't in Num. So, how do I provoke a type check error in this case? Thanks in advance.

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