September 2010 - Haskell-Cafe

Re: [Haskell-cafe] Restricted type classes
by John Lato 07 Sep '10

07 Sep '10

> > From: Stephen Tetley <stephen.tetley(a)gmail.com> > > On 6 September 2010 20:18, John Lato <jwlato(a)gmail.com> wrote: > > > Can you give an example of a Functor that doesn't have pure? I think > it's > > Pointed Functors which are useful; not Functor by itself. > > Strictly speaking is Pair one? The current implementation tacks on monoid. > Interesting example. I tend not to like implementations which rely on bottoms, so I think I'd agree. In a similar spirit, I would propose Data.Vector.Unboxed.Vector as a Pointed Functor (modulo element restrictions) which is not Applicative, since the necessary element restrictions mean <*> can't be defined. John

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Oracle stored procedures
by Peter Marks 07 Sep '10

07 Sep '10

Are there any Haskell libraries that can call stored procedures in Oracle? I've been looking at Takusen which I like, but I can't find a way to call a stored procedure. Thanks Peter

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Re: [Haskell-cafe] container-classes (was: Restricted type classes)
by John Lato 06 Sep '10

06 Sep '10

I'd like to make one more argument in favor of my preference for more splitting of type classes. IMO it's beneficial to split up classes to minimize unnecessary dependencies. That is, while e.g. Monoid is very useful for containers, many container methods won't need it, e.g. "elem" or "filter". When you divide the type classes so only what's necessary is required, more data structures will be able to meet those criteria, which in turn makes the class more generally useful. As an example, I have an extension to iteratee which uses raw buffers for holding data. This allows it to operate in truly constant memory (only one allocation, deallocated when it's finished), which has resulted in performance benefits. Unfortunately there is not a good Monoid instance for this type because appending buffers is not possible. By introducing the NullPoint class (equivalent to mempty of Monoid) in iteratee, my raw buffer extension is able to re-use several iteratee functions which it would not be able to if I had require Monoid instead of NullPoint. I think I've made my arguments as well as I can, so I'm going to be quiet now unless something new comes up. Cheers, John

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overloaded list literals?
by Johannes Waldmann 06 Sep '10

06 Sep '10

I think left-biased (= singly linked) lists are much overrated in Haskell coding (and teaching). The language (syntax and Prelude) makes it just too easy to use them, and old habits (from LISP) die hard. Sure, lists serve a purpose: * they model (infinite, lazy) streams, used in the producer/transformer/consumer pattern * they are an algebraic data type, so you can use them to teach recursion ((co-)induction); but more often, lists are (mis-)used when actually * you want some efficiently index-able and concat-able sequence type * or you don't need the indexing, just membership, so you actually want Data.Set (disregarding strictness and unwanted Ord instances for the moment). It is an empirical law that in 90 percent of the cases where a computer science student says "list" he means "set". * you avoid/forget to tell your students about algebraic data types in general. Hypothetically now ... We have overloaded numerical literals (Num.fromInteger) and we can overload string literals (IsString.fromString), so how about using list syntax ( [], : ) for anything list-like (e.g., Data.Sequence)? Of course some "minor details" would need to be worked out, like what methods should go in the hypothetical "class IsList" (is is Foldable?) and what to do about pattern matching (perhaps we don't need it?) IIRC there was a time when list comprehension would actually mean monad comprehension (when there was no "do" notation) but that's not what I'm getting at here. Or is it? Do we have a "Haskell museum" of ideas from the past? Best - J.W.

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Projects that could use student contributions?
by Brent Yorgey 06 Sep '10

06 Sep '10

Hi all, This fall I'll be teaching a half-credit introduction to Haskell to some undergrads. As a final project I am thinking of giving them the option of (instead of developing some program/project of their own) contributing to an existing open-source Haskell project. Of course, this requires the existence of projects they could contribute to. I'm sure they exist, but need your help to figure out what they are. So, do you maintain, or know of, any projects with the following characteristics? * might conceivably be interesting to undergraduate CS majors * simple enough that someone could make some non-trivial contributions in the space of 3 or 4 weeks * could use some help! This is a little non-traditional, so we'll see how it goes! -Brent

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running ghci simply
by Johann Bach 06 Sep '10

06 Sep '10

Is there a way to run a Haskell module via ghci with a single command on the command-line? Note: I'm on Windows XP. If I have the module test.hs, I can type "test.hs" and that will load it into ghci. However, I need to then type "main" to run it, and then manually exit ghci. I would like to do all this in one step.

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ANN: hledger 0.12.1
by Simon Michael 06 Sep '10

06 Sep '10

I'm pleased to announce hledger 0.12.1, with a new web interface and bugfixes. Thanks to Ben Boeckel and David Patrick for their help this time around. Installation docs, linux/mac/windows binaries and more are at http://hledger.org and http://hackage.haskell.org/package/ hledger . Release notes: 2010/9/6 hledger 0.12.1 * web: new, better web ui; accounts are now a permanent sidebar; add form uses auto-completing combo fields * installing: fix a build error with parsec 3 (#22) * installing: require exactly matching hledger-lib version for more robust builds * installing: explicit data-object dependency to ensure hledger and hledger-lib use the same time version * installing: explicit hamlet dependency for more robust building * installing: build threaded and with warnings * installing: drop -fweb610 flag * installing: add gtk2hs-buildtools dependency needed to build with -fchart * installing: require cabal 1.6 or greater * add -D/--daily flag * register: with --depth, clip account names or aggregate postings rather than excluding them * fix !include with deeply nested directories (#21) * fix obscured date parse errors with parsec 3 * handle unicode better in errors * fix a ghc 6.12.3 error when running interpreted Stats: 50 days and 90 commits since last release, now at 5741 lines of code with 136 tests and 41% unit test coverage. Goals for the next release are: 1. implement a plugin mechanism, probably by searching for hledger-* named binaries in $PATH at startup and handing over control. This should make packaging easier and more scalable. 2. move web/vty/chart features into their own packages. 3. developer docs 4. a public project ledger tracking expenses/income/resources Problem reports and all help are welcome; additional bugfix releases will appear on hackage if needed. Best, -Simon

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Graphics.Drawing
by han 06 Sep '10

06 Sep '10

I think there should be Graphics.Drawing (along with Graphics.Rendering) and many graphics packages should go into it. So * Graphics.Rendering will contain more technical and rendering-engine-level packages (OpenGL, GD, ...) * while Graphics.Drawing will be for higher-level and user-friendlier drawing tools. (gloss, graphics-drawingcombinators, hieroglyph, Chart, ...) Currently most graphics packages are thrown into Graphics.Rendering or even Graphics. I think we all agree that having a standalone package right under Graphics is undesirable. There needs to be at least one subcategory. (which is why I also think Win32 and X11 should be moved from Graphics to Graphics.UI.) What do you think?

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Update gtk2hs!
by Andy Stewart 06 Sep '10

06 Sep '10

Hi all, I'm working on update gtk2hs APIs. 'gio' has update to newest version, all patches has push to repo, i need more test before release gio-0.12.0 About `gtk` packages, i have push some gtk+-2.18/gtk+-2.20 patches to repo but not all, i plan finish all APIs before release gtk-0.12.0. If anyone interested this, welcome to join! :) We can release gtk-0.12.0 quicker if more people join us. GTK+-2.18 new modules have *finish* at http://www2.in.tum.de/~simona/gtk2hs-2.18/gtk/Graphics/UI/Gtk/ Because have some bugs in those code (my fault), you need review every APIs before send patch, but it's much simple than build those module by yourself. Below are missing modules in gtk+-2.18: gtk/Graphics/UI/Gtk/Printing/PageSetup.chs gtk/Graphics/UI/Gtk/Printing/PaperSize.chs gtk/Graphics/UI/Gtk/Printing/PrintContext.chs gtk/Graphics/UI/Gtk/Printing/PrintOperation.chs gtk/Graphics/UI/Gtk/Printing/PrintOperationPreview.chs gtk/Graphics/UI/Gtk/Printing/PrintSettings.chs gtk/Graphics/UI/Gtk/Recent/RecentChooser.chs gtk/Graphics/UI/Gtk/Recent/RecentChooserDialog.chs gtk/Graphics/UI/Gtk/Recent/RecentChooserMenu.chs gtk/Graphics/UI/Gtk/Recent/RecentChooserWidget.chs gtk/Graphics/UI/Gtk/Recent/RecentFilter.chs gtk/Graphics/UI/Gtk/Recent/RecentInfo.chs gtk/Graphics/UI/Gtk/Recent/RecentManager.chs gtk/Graphics/UI/Gtk/Special/HRuler.chs gtk/Graphics/UI/Gtk/Special/Ruler.chs gtk/Graphics/UI/Gtk/Special/VRuler.chs gtk/Graphics/UI/Gtk/Selectors/HSV.chs gtk/Graphics/UI/Gtk/ActionMenuToolbar/Action.chs gtk/Graphics/UI/Gtk/ActionMenuToolbar/ActionGroup.chs gtk/Graphics/UI/Gtk/ActionMenuToolbar/Activatable.chs gtk/Graphics/UI/Gtk/ActionMenuToolbar/RadioAction.chs gtk/Graphics/UI/Gtk/ActionMenuToolbar/RecentAction.chs gtk/Graphics/UI/Gtk/ActionMenuToolbar/ToggleAction.chs gtk/Graphics/UI/Gtk/ActionMenuToolbar/UIManager.chs gtk/Graphics/UI/Gtk/Buttons/gtk/Graphics/UI/Gtk/LinkButton.chs gtk/Graphics/UI/Gtk/Buttons/gtk/Graphics/UI/Gtk/ScaleButton.chs gtk/Graphics/UI/Gtk/Buttons/gtk/Graphics/UI/Gtk/VolumeButton.chs gtk/Graphics/UI/Gtk/Display/InfoBar.chs gtk/Graphics/UI/Gtk/Entry/EntryBuffer.chs gtk/Graphics/UI/Gtk/Gdk/AppLaunchContext.chs gtk/Graphics/UI/Gtk/Gdk/Colormap.chs gtk/Graphics/UI/Gtk/Gdk/Device.chs gtk/Graphics/UI/Gtk/Gdk/DisplayManager.chs gtk/Graphics/UI/Gtk/Gdk/Keymap.chs gtk/Graphics/UI/Gtk/Gdk/PixbufLoader.chs gtk/Graphics/UI/Gtk/Gdk/Visual.chs gtk/Graphics/UI/Gtk/General/AccelGroup.chs gtk/Graphics/UI/Gtk/General/AccelMap.chs gtk/Graphics/UI/Gtk/General/Binding.chs gtk/Graphics/UI/Gtk/General/IconInfo.chs gtk/Graphics/UI/Gtk/General/Paint.chs gtk/Graphics/UI/Gtk/MenuComboToolbar/Item.chs gtk/Graphics/UI/Gtk/MenuComboToolbar/ToolShell.chs gtk/Graphics/UI/Gtk/Misc/IMContextSimple.chs gtk/Graphics/UI/Gtk/ModelView/CellEditable.chs gtk/Graphics/UI/Gtk/ModelView/CellRendererAccel.chs gtk/Graphics/UI/Gtk/ModelView/CellRendererSpin.chs GTK+2.20 just below APIs haven't finish: Index of new symbols in 2.20 C GtkCellEditable:editing-canceled, object property in GtkCellEditable GtkCellRendererSpinner:pulse, object property in GtkCellRendererSpinner GtkCellRendererSpinner:size, object property in GtkCellRendererSpinner gtk_cell_renderer_spinner_new, function in GtkCellRendererSpinner O gtk_offscreen_window_get_pixbuf, function in GtkOffscreenWindow gtk_offscreen_window_get_pixmap, function in GtkOffscreenWindow gtk_offscreen_window_new, function in GtkOffscreenWindow P gtk_paint_spinner, function in GtkStyle gtk_printer_get_hard_margins, function in GtkPrinter gtk_print_context_get_hard_margins, function in GtkPrintContext T GtkToolPalette::set-scroll-adjustments, object signal in GtkToolPalette GtkToolPalette:exclusive, object property in GtkToolPalette GtkToolPalette:expand, object property in GtkToolPalette GtkToolPalette:icon-size, object property in GtkToolPalette GtkToolPalette:icon-size-set, object property in GtkToolPalette GtkToolPalette:toolbar-style, object property in GtkToolPalette gtk_tool_item_get_ellipsize_mode, function in GtkToolItem gtk_tool_item_get_text_alignment, function in GtkToolItem gtk_tool_item_get_text_orientation, function in GtkToolItem gtk_tool_item_get_text_size_group, function in GtkToolItem gtk_tool_item_group_get_collapsed, function in GtkToolItemGroup gtk_tool_item_group_get_drop_item, function in GtkToolItemGroup gtk_tool_item_group_get_ellipsize, function in GtkToolItemGroup gtk_tool_item_group_get_header_relief, function in GtkToolItemGroup gtk_tool_item_group_get_item_position, function in GtkToolItemGroup gtk_tool_item_group_get_label, function in GtkToolItemGroup gtk_tool_item_group_get_label_widget, function in GtkToolItemGroup gtk_tool_item_group_get_nth_item, function in GtkToolItemGroup gtk_tool_item_group_get_n_items, function in GtkToolItemGroup gtk_tool_item_group_insert, function in GtkToolItemGroup gtk_tool_item_group_new, function in GtkToolItemGroup gtk_tool_item_group_set_collapsed, function in GtkToolItemGroup gtk_tool_item_group_set_ellipsize, function in GtkToolItemGroup gtk_tool_item_group_set_header_relief, function in GtkToolItemGroup gtk_tool_item_group_set_item_position, function in GtkToolItemGroup gtk_tool_item_group_set_label, function in GtkToolItemGroup gtk_tool_item_group_set_label_widget, function in GtkToolItemGroup gtk_tool_palette_add_drag_dest, function in GtkToolPalette gtk_tool_palette_get_drag_item, function in GtkToolPalette gtk_tool_palette_get_drag_target_group, function in GtkToolPalette gtk_tool_palette_get_drag_target_item, function in GtkToolPalette gtk_tool_palette_get_drop_group, function in GtkToolPalette gtk_tool_palette_get_drop_item, function in GtkToolPalette gtk_tool_palette_get_exclusive, function in GtkToolPalette gtk_tool_palette_get_expand, function in GtkToolPalette gtk_tool_palette_get_group_position, function in GtkToolPalette gtk_tool_palette_get_hadjustment, function in GtkToolPalette gtk_tool_palette_get_icon_size, function in GtkToolPalette gtk_tool_palette_get_style, function in GtkToolPalette gtk_tool_palette_get_vadjustment, function in GtkToolPalette gtk_tool_palette_new, function in GtkToolPalette gtk_tool_palette_set_drag_source, function in GtkToolPalette gtk_tool_palette_set_exclusive, function in GtkToolPalette gtk_tool_palette_set_expand, function in GtkToolPalette gtk_tool_palette_set_group_position, function in GtkToolPalette gtk_tool_palette_set_icon_size, function in GtkToolPalette gtk_tool_palette_set_style, function in GtkToolPalette gtk_tool_palette_unset_icon_size, function in GtkToolPalette gtk_tool_palette_unset_style, function in GtkToolPalette After finish above modules, gtk2hs will support all APIs to gtk+-2.20! Any suggestion and bug repot are welcome! Cheers, -- Andy

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Operator precedence
by michael rice 06 Sep '10

06 Sep '10

Is there a handy list of operators and their precedence somewhere? Michael

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