- Haskell-Cafe - Haskell.org

testing for exceptions
by Roelof Wobben 02 Nov '15

02 Nov '15

Hello, I have made this exercise which can be found at the Craft of Functional Programming book. -- exercise 32 -- Suppose we have to raise 2 to the power n. If n is even, 2*m say, then -- 2n = 22*m = (2m)2 -- If n is odd, 2*m+l say, then -- 2n = 22*m+l = (2n)2*2 -- Give a recursive function to compute 2n which uses these insights. f2 :: Integer -> Integer f2 n | n < 0 = error "This will only run for positive numbers" | n == 0 = 1 | even n = f2 ( n `div` 2) ^ 2 | odd n = (f2 ( n `div` 2) ^ 2) * 2 Now I have to make Hunit tests for it, But is there a way I can test if the error message is shown when a negative number is being used ? Roelof

8 18

what do I have to do exactlry with this exercises
by Roelof Wobben 02 Nov '15

02 Nov '15

Hello, Im self studing Haskell with the Craft o ffunctional programmimg of Hutton. Now I see two exercises that I do not understand what is really the purpose here. The two exercises are : 4.21 Given a function f of type Integer -> Integer give a recursive definition of a function of type Integer -> Integer which on input n returns the maximum of the values f 0, f 1, ..., f n. You might find the max function defined in Section 3.4 useful. To test this function, add to your script a definition of some values of f thus: f 0 = 0 f 1 = 44 f 2 = 17 f _ = 0 and so on; then test your function at various values. 4.22 Given a function f of type Integer -> Integer give a recursive definition of a function of type Integer -> Bool which on input n returns True if one or more of the values f 0, f 1, ..., f n is zero and False otherwise.

3 10

Find a point inside (x,y,z) -> Bool
by martin 02 Nov '15

02 Nov '15

Hello all, I hope this is not a too silly question. It goes like this: Suppose I have a shape defined as (x,y,z) -> Bool how can I find a Point inside this shape? Obviously I could iterate through all possible x,y and z, but this appears very expensive. There may be no point at all at x=0. With brute force iteration I would have no clue that the False I am receiving with (0,1,1) is caused by x=0 and I may nedlessly try all combinations of y and z without ever receiving a True. Are there any alternative ways of finding points inside a shape?

11 16

Re: [Haskell-cafe] ANN: hledger-0.27, with more console power
by Simon Michael 02 Nov '15

02 Nov '15

Hi Johannes, quite right. I've added a few screenshots to the manual at http://hledger.org/manual#ui <http://hledger.org/manual#ui> . Some of them use the more realistic example data from the beancount project, thanks to Martin Blais. Best, -Simon > On Nov 1, 2015, at 10:16 AM, johannesjh(a)gmail.com wrote: > > Hi Simon! > > The availability of a text-based user interface for hledger sounds great. Could you please post some screenshots of what the UI looks like? > > thank you very much, > Johannes > > > > On Saturday, October 31, 2015 at 10:38:20 PM UTC+1, Simon Michael (sm) wrote: > I'm pleased to announce hledger 0.27! > > What is it ? > hledger is a cross-platform program for tracking money, time, or any > other commodity, using double-entry accounting and a simple, editable > file format. It can also read CSV or timelog files, and export CSV. > It provides command-line, curses and web interfaces, and aims to be a > reliable, practical tool for daily use. It is inspired by and largely > compatible with ledger(1). > > What's new ? > This release introduces hledger-ui, a new curses-style interface that > I'm quite pleased with (not yet available on Windows). Built on vty > and the new brick library, it lets you review account balances and > transactions with fewer keystrokes and less effort. Ledger users with > compatible journal files may also find this useful. > > hledger can now report current value based on market prices (hurrah); > wide characters are displayed properly; regular expression account > aliases are fast; and unix man pages are provided. You can see all > changes and bugfixes at http://hledger.org/release-notes#hledger-0.27 <http://hledger.org/release-notes#hledger-0.27> . > > Release contributors: > Simon Michael, > Carlos Lopez-Camey. > > How to get it ? > hledger can be installed with cabal or stack - see > http://hledger.org/download <http://hledger.org/download> for guidance. > If you'd like to contribute Windows or Mac binaries, please get in > touch. > > Happy Hallowe'en! > -Simon > > > -- > You received this message because you are subscribed to the Google Groups "hledger" group. > To unsubscribe from this group and stop receiving emails from it, send an email to hledger%2Bunsubscribe(a)googlegroups.com <mailto:hledger+unsubscribe@googlegroups.com>. > For more options, visit https://groups.google.com/d/optout <https://groups.google.com/d/optout>.

1 0

Month in Haskell Mode October 2015
by Gracjan Polak 01 Nov '15

01 Nov '15

Welcome Haskell Mode users, Haskell Mode progress report for October 2015. It is also available online <https://github.com/haskell/haskell-mode/wiki/Month-in-Haskell-Mode-October-…> and on reddit <https://www.reddit.com/r/haskell/comments/3r26px/month_in_haskell_mode_octo…> . For previous issue see September 2015 <https://github.com/haskell/haskell-mode/wiki/Month-in-Haskell-Mode-Septembe…> . <https://github.com/haskell/haskell-mode/wiki/Month-in-Haskell-Mode-October-…>What is Haskell Mode? Haskell Mode is an umbrella project for multiple Emacs tools for efficient Haskell development. Haskell Mode is an open source project developed by a group of volunteers. For more information see https://github.com/haskell/haskell-mode. <https://github.com/haskell/haskell-mode/wiki/Month-in-Haskell-Mode-October-…>Important developments Functionality provided by haskell-indentation received a lot of fixes and contributions lifting it to a complete new level of quality. haskell-indentation is now a default indentation mode enabled when haskell-mode itself is enabled. ATTENTION TUTORIAL WRITERS: The following line is NO LONGER NEEDED: (add-hook 'haskell-mode-hook 'haskell-indentation-mode) PLEASE REMOVE IT FROM ALL THE TUTORIALS OUT THERE. Thanks. Note that for backward compatibility all other indentation modes know how to properly handle new defaults. We have also improved our development process by defining Complete functionality <https://github.com/haskell/haskell-mode/wiki/Complete-functionality> as a concept. <https://github.com/haskell/haskell-mode/wiki/Month-in-Haskell-Mode-October-…>Cooperation with related projects As noted last time Haskell Mode wanted to reach out to other projects in editor space. This has not happened because even better events unfolded. Things of note: haskell-ide-(backend) <https://github.com/haskell/haskell-ide> project got started by Alan Zimmerman. The goal is to bring in all the backend tools that editors need to properly handle advanced Haskell editing. ghc-mod <https://github.com/kazu-yamamoto/ghc-mod> does not need to be introduced. We are thinking how to better split work between haskell-mode and ghc-mod. HaRe <https://github.com/RefactoringTools/HaRe> is a Haskell refactorer. It can rename all the one letter variables to something more descriptive! It can also do other things. And it has somewhat working Emacs integration. <https://github.com/haskell/haskell-mode/wiki/Month-in-Haskell-Mode-October-…>Heads up tutorial writers! There are plenty tutorials out there how to setup haskell-mode. Thanks for writing those! Note that some of them are quite outdated at this point in time and a bit confusing due to the improvements in haskell-mode. Can I ask tutorial writers to contribute directly to haskell mode documentation so that Haskell Mode Manual can be kept up to date with haskell-mode developments? Please contribute documentation back to the project. External tutorials get out of hand pretty fast. Thanks <https://github.com/haskell/haskell-mode/wiki/Month-in-Haskell-Mode-October-…>Heads up! Cruft removal! haskell-mode is about to loose some weight so that it can be better carried by Haskell Mode Community. On the list to be removed are: - Remove haskell-bot.el <https://github.com/haskell/haskell-mode/pull/960> - Remove scc functions <https://github.com/haskell/haskell-mode/pull/961> - Remove horizontal whitespace based smart indentation mode haskell-simple-indent <https://github.com/haskell/haskell-mode/pull/958> - Remove Unicode input method <https://github.com/haskell/haskell-mode/pull/939> - Remove haskell-checkers.el <https://github.com/haskell/haskell-mode/pull/959> If you use one of the above yourself, feel strongly that it should be supported by the community and volunteer to write some unit tests and documentation, please speak up! The changes above will be merged no earlier than December 2015 so there is time to chime in. <https://github.com/haskell/haskell-mode/wiki/Month-in-Haskell-Mode-October-…>Current project focus Current project focus is to lower entry barrier for newcomers by defining bite-sized tasks. Get 50 'well-defined-tasks' done as by the metric: https://github.com/haskell/haskell-mode/issues?q=is%3Aissue+label%3Awell-de… A 'well-defined-task' is a category of tasks that have the field cleared for them, questions already sorted out and detailed information how to get them done. So you can just sit, search for 'well-defined-task' label and enjoy the coding! The point is to lower the entry barrier for new users, new issue reporters and advanced programmers but Emacs lisp beginners to contribute to the project. Current status: 13 well-defined-tasks closed <https://github.com/haskell/haskell-mode/issues?q=is%3Aissue+label%3Awell-de…> plus 13 more open <https://github.com/haskell/haskell-mode/issues?q=is%3Aissue+label%3Awell-de…> . If only you can help with reaching our targets please do so! <https://github.com/haskell/haskell-mode/wiki/Month-in-Haskell-Mode-October-…>Issues closed in October - Disable Indentation #90 <https://github.com/haskell/haskell-mode/issues/90> - Unicode symbols inside ghci prompt hang inferior-haskell-mode #176 <https://github.com/haskell/haskell-mode/issues/176> - Multi-line -ferror-spans syntax not supported #192 <https://github.com/haskell/haskell-mode/issues/192> - [Discussion] Add end-to-end guide for configuring haskell-mode in a place where users will see it #197 <https://github.com/haskell/haskell-mode/issues/197> - Sorting imported functions from a module alphabetically #206 <https://github.com/haskell/haskell-mode/issues/206> - After the newline have to hit twice to start cycling #276 <https://github.com/haskell/haskell-mode/issues/276> - indent-according-to-mode opens the help buffer for haskell-mode-hook #290 <https://github.com/haskell/haskell-mode/issues/290> - Can't get type info unless file compiles #294 <https://github.com/haskell/haskell-mode/issues/294> - Parsing broken on quasi quotes #323 <https://github.com/haskell/haskell-mode/issues/323> - getLine behaves not correctly in Interactive-Haskell-mode. #333 <https://github.com/haskell/haskell-mode/issues/333> - Overly aggressive indenting after recent Emacs update #377 <https://github.com/haskell/haskell-mode/issues/377> - haskell-indentation-mode parse error on indenting import declaration #397 <https://github.com/haskell/haskell-mode/issues/397> - Pick one indentation mode as preferred in the documentation/customize. #414 <https://github.com/haskell/haskell-mode/issues/414> - haskell-indendation and guards #481 <https://github.com/haskell/haskell-mode/issues/481> - Running indent-according-to-mode moves the cursor way to the right #488 <https://github.com/haskell/haskell-mode/issues/488> - haskell-session-change-target should list available targets #534 <https://github.com/haskell/haskell-mode/issues/534> - Haskell-mode crashing and buffer erased (not sure exactly where problem is) #594 <https://github.com/haskell/haskell-mode/issues/594> - haskell-indentation breaks with UnicodeSyntax #715 <https://github.com/haskell/haskell-mode/issues/715> - Handle Template Haskell / DataKinds names in haskell-indentation #760 <https://github.com/haskell/haskell-mode/issues/760> - No indentation suggested for case clauses #786 <https://github.com/haskell/haskell-mode/issues/786> - Pull in ghc-mod's ghc-make-indent-shallower/deeper into haskell-mode #826 <https://github.com/haskell/haskell-mode/issues/826> - Clean up example init.el on wiki #843 <https://github.com/haskell/haskell-mode/issues/843> - Indentation mode needs to be able to recover from parser errors #861 <https://github.com/haskell/haskell-mode/issues/861> - haskell-simple-indent-mode non-responsive #867 <https://github.com/haskell/haskell-mode/issues/867> - Get hit by 'Invalid Token := )' #875 <https://github.com/haskell/haskell-mode/issues/875> - Invalid Token :| #878 <https://github.com/haskell/haskell-mode/issues/878> - Parse error: Illegal token: where #883 <https://github.com/haskell/haskell-mode/issues/883> - parse-error "Expecting then" "Expecting else" DoAndIfThenElse in indentation #884 <https://github.com/haskell/haskell-mode/issues/884> - Illegal tokens and QuasiQuotes #894 <https://github.com/haskell/haskell-mode/issues/894> - Haskell Interactive Shell Stuck in Insert Mode (spacemacs) #895 <https://github.com/haskell/haskell-mode/issues/895> - Syntax highlighting: no difference between data constructor and type ? #898 <https://github.com/haskell/haskell-mode/issues/898> - Let the enter create a new line despite the indentation errors #905 <https://github.com/haskell/haskell-mode/issues/905> - Parse Error inside QuasiQuotes #907 <https://github.com/haskell/haskell-mode/issues/907> - "Expecting else" error message #917 <https://github.com/haskell/haskell-mode/issues/917> - Support commas in first indentation column #918 <https://github.com/haskell/haskell-mode/issues/918> - haskell-process changes the directory which causes loaded modules to be unloaded #923 <https://github.com/haskell/haskell-mode/issues/923> - Syntax highlighting only #924 <https://github.com/haskell/haskell-mode/issues/924> - haskell-process.el binds C-c c but should not #926 <https://github.com/haskell/haskell-mode/issues/926> - Simple indent vs haskell-indentation when using autofill on comments #932 <https://github.com/haskell/haskell-mode/issues/932> - How to use REPL with stack? #953 <https://github.com/haskell/haskell-mode/issues/953> - Wiki documentation out of sync ? #968 <https://github.com/haskell/haskell-mode/issues/968> ### Pull requests merged in October - Support where keyword after any expression #897 <https://github.com/haskell/haskell-mode/pull/897> - Support names starting with quotes #903 <https://github.com/haskell/haskell-mode/pull/903> - Fix switching to the presentation buffer #904 <https://github.com/haskell/haskell-mode/pull/904> - Implement QuasiQuotes syntax in indentation #909 <https://github.com/haskell/haskell-mode/pull/909> - Add build notification to #haskell-emacs #912 <https://github.com/haskell/haskell-mode/pull/912> - Mobile friendly README.md #913 <https://github.com/haskell/haskell-mode/pull/913> - Add failing tests for indentation #916 <https://github.com/haskell/haskell-mode/pull/916> - Toplevel indentation bugfixes for data, newtype, instance #919 <https://github.com/haskell/haskell-mode/pull/919> - Document failing indentation #920 <https://github.com/haskell/haskell-mode/pull/920> - Navigate to top level declaration in indenatation #921 <https://github.com/haskell/haskell-mode/pull/921> - Make haskell indentation the default indentation mode #922 <https://github.com/haskell/haskell-mode/pull/922> - Don't change directory when using stack-ghci. #925 <https://github.com/haskell/haskell-mode/pull/925> - Add rather baroque indentation test which cause parse failure #927 <https://github.com/haskell/haskell-mode/pull/927> - Yet another parser failure #928 <https://github.com/haskell/haskell-mode/pull/928> - Reloading: haskell-process-file-loadish needs the current buffer as an argument #929 <https://github.com/haskell/haskell-mode/pull/929> - Fix nested layout 1 #930 <https://github.com/haskell/haskell-mode/pull/930> - Filter only sections bearing targets - fixes #534 #931 <https://github.com/haskell/haskell-mode/pull/931> - Honour curly braces in target sections #933 <https://github.com/haskell/haskell-mode/pull/933> - Add failing tests for multiline string literals #934 <https://github.com/haskell/haskell-mode/pull/934> - Mark some indentation tests as passing #936 <https://github.com/haskell/haskell-mode/pull/936> - Remove useless debugging functions #937 <https://github.com/haskell/haskell-mode/pull/937> - Write tests properly, mark them as passing #944 <https://github.com/haskell/haskell-mode/pull/944> - Remove auto-fill code from indentation #945 <https://github.com/haskell/haskell-mode/pull/945> - Improve can grab prefix for completions #946 <https://github.com/haskell/haskell-mode/pull/946> - Remove dynamic indentation showing #947 <https://github.com/haskell/haskell-mode/pull/947> - Indentation of guards with commas is working properly #948 <https://github.com/haskell/haskell-mode/pull/948> - Fix indent for do inside a list #956 <https://github.com/haskell/haskell-mode/pull/956> - Recognize top level constructs in indentation #965 <https://github.com/haskell/haskell-mode/pull/965> - Remove indentation safety hack #966 <https://github.com/haskell/haskell-mode/pull/966> - Change 'w.r.t' to 'with regards to' for clarity #967 <https://github.com/haskell/haskell-mode/pull/967> <https://github.com/haskell/haskell-mode/wiki/Month-in-Haskell-Mode-October-…>Contributors active in October Alexey Khudyakov, Chris Done, Gracjan Polak, Gregor Riegler, Kabelo Moiloa, Marlo Major, vlatkoB <https://github.com/haskell/haskell-mode/wiki/Month-in-Haskell-Mode-October-…> Contributing Haskell Mode needs volunteers like any other open source project. For more information see: https://github.com/haskell/haskell-mode/wiki Also drop by our IRC channel: #haskell-emacs at irc.freenode.net. Thanks!

1 0

Re: [Haskell-cafe] testing for exceptions
by Darren Grant 01 Nov '15

01 Nov '15

Great points. I often run into situations where sketching out non-terminating endpoints leads to a precise domain, but after peer review it becomes clear that a much simpler program would result from compromising with a less precise domain that eliminates these points altogether. Eliminating branching on constructors in Haskell is a common aspect of this refinement, it's true. Cheers, Darren On Nov 1, 2015 03:18, <joachifm(a)fastmail.fm> wrote: The ability to express non-termination is a feature, not a bug. If the program truly cannot produce a useful result for some input, it should crash, the earlier the better. Wrapping the return value ONLY to make a non-total program *appear* total is kind of ugly (and commits you to a potentially inappropriate/nonsensical model for the problem at hand). It is cleaner to either constrain the input to values you're prepared to deal with or crash when the implicit invariant is violated. _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe

1 0

ANN: hledger-0.27, with more console power
by Simon Michael 01 Nov '15

01 Nov '15

I'm pleased to announce hledger 0.27! What is it ? hledger is a cross-platform program for tracking money, time, or any other commodity, using double-entry accounting and a simple, editable file format. It can also read CSV or timelog files, and export CSV. It provides command-line, curses and web interfaces, and aims to be a reliable, practical tool for daily use. It is inspired by and largely compatible with ledger(1). What's new ? This release introduces hledger-ui, a new curses-style interface that I'm quite pleased with (not yet available on Windows). Built on vty and the new brick library, it lets you review account balances and transactions with fewer keystrokes and less effort. Ledger users with compatible journal files may also find this useful. hledger can now report current value based on market prices (hurrah); wide characters are displayed properly; regular expression account aliases are fast; and unix man pages are provided. You can see all changes and bugfixes at http://hledger.org/release-notes#hledger-0.27 . Release contributors: Simon Michael, Carlos Lopez-Camey. How to get it ? hledger can be installed with cabal or stack - see http://hledger.org/download for guidance. If you'd like to contribute Windows or Mac binaries, please get in touch. Happy Hallowe'en! -Simon

1 0

Re: [Haskell-cafe] Determining the length of a Foldable Applicative.
by David Banas 31 Oct '15

31 Oct '15

Thanks to all, whom responded to this! With your help, I found my way to this, which seems to work correctly: app_len :: (Foldable f) => f a -> Int app_len = foldl' (flip ((+) . const 1)) 0 -db On Oct 30, 2015, at 2:47 PM, David Kraeutmann <kane(a)kane.cx> wrote: > Your types don't match here, but you have the right idea. > > 'sum' won't be enough. Take a look at the definition of sum: > >> sum :: (Foldable t, Num a) => t a -> a >> sum = getSum . foldMap Sum > > That won't work, since that requires '(Num a)'. > > We just want to count 1 up whenever we map an element. > Try doing something with const and Data.Monoid.Sum. > > > On 10/30/2015 9:03 PM, David Banas wrote: >> Hi all, >> >> I thought I had a simple way to determine the “length" (i.e. - number of elements in) of a Foldable Applicative container: >> >> import Prelude hiding (sum) >> import Data.Foldable (Foldable(..), sum) >> import Control.Applicative >> >> -- Calculate the "length" (i.e. - number of elements in) an Applicative container. >> app_len :: (Applicative f, Foldable f) => f a -> Int >> app_len = sum $ pure 1 >> >> but I didn’t: >> >> app_len_test.hs:9:11: >> Could not deduce (Foldable t0) arising from a use of ‘sum’ >> from the context (Applicative f, Foldable f) >> bound by the type signature for >> app_len :: (Applicative f, Foldable f) => f a -> Int >> at app_len_test.hs:8:12-52 >> The type variable ‘t0’ is ambiguous >> Note: there are several potential instances: >> instance Foldable ((,) a) -- Defined in ‘Data.Foldable’ >> instance GHC.Arr.Ix i => Foldable (GHC.Arr.Array i) >> -- Defined in ‘Data.Foldable’ >> instance Foldable (Const m) -- Defined in ‘Data.Foldable’ >> ...plus four others >> In the expression: sum >> In the expression: sum $ pure 1 >> In an equation for ‘app_len’: app_len = sum $ pure 1 >> >> app_len_test.hs:9:17: >> Could not deduce (Applicative t0) arising from a use of ‘pure’ >> from the context (Applicative f, Foldable f) >> bound by the type signature for >> app_len :: (Applicative f, Foldable f) => f a -> Int >> at app_len_test.hs:8:12-52 >> The type variable ‘t0’ is ambiguous >> Note: there are several potential instances: >> instance Data.Monoid.Monoid a => Applicative ((,) a) >> -- Defined in ‘Control.Applicative’ >> instance Applicative ((->) a) -- Defined in ‘Control.Applicative’ >> instance Control.Arrow.Arrow a => >> Applicative (Control.Arrow.ArrowMonad a) >> -- Defined in ‘Control.Applicative’ >> ...plus 14 others >> In the second argument of ‘($)’, namely ‘pure 1’ >> In the expression: sum $ pure 1 >> In an equation for ‘app_len’: app_len = sum $ pure 1 >> >> Can anyone help me understand what I’m missing? >> >> Thanks, and have a great weekend, >> -db >> >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe >> >

1 0

Determining the length of a Foldable Applicative.
by David Banas 31 Oct '15

31 Oct '15

Hi all, I thought I had a simple way to determine the “length" (i.e. - number of elements in) of a Foldable Applicative container: import Prelude hiding (sum) import Data.Foldable (Foldable(..), sum) import Control.Applicative -- Calculate the "length" (i.e. - number of elements in) an Applicative container. app_len :: (Applicative f, Foldable f) => f a -> Int app_len = sum $ pure 1 but I didn’t: app_len_test.hs:9:11: Could not deduce (Foldable t0) arising from a use of ‘sum’ from the context (Applicative f, Foldable f) bound by the type signature for app_len :: (Applicative f, Foldable f) => f a -> Int at app_len_test.hs:8:12-52 The type variable ‘t0’ is ambiguous Note: there are several potential instances: instance Foldable ((,) a) -- Defined in ‘Data.Foldable’ instance GHC.Arr.Ix i => Foldable (GHC.Arr.Array i) -- Defined in ‘Data.Foldable’ instance Foldable (Const m) -- Defined in ‘Data.Foldable’ ...plus four others In the expression: sum In the expression: sum $ pure 1 In an equation for ‘app_len’: app_len = sum $ pure 1 app_len_test.hs:9:17: Could not deduce (Applicative t0) arising from a use of ‘pure’ from the context (Applicative f, Foldable f) bound by the type signature for app_len :: (Applicative f, Foldable f) => f a -> Int at app_len_test.hs:8:12-52 The type variable ‘t0’ is ambiguous Note: there are several potential instances: instance Data.Monoid.Monoid a => Applicative ((,) a) -- Defined in ‘Control.Applicative’ instance Applicative ((->) a) -- Defined in ‘Control.Applicative’ instance Control.Arrow.Arrow a => Applicative (Control.Arrow.ArrowMonad a) -- Defined in ‘Control.Applicative’ ...plus 14 others In the second argument of ‘($)’, namely ‘pure 1’ In the expression: sum $ pure 1 In an equation for ‘app_len’: app_len = sum $ pure 1 Can anyone help me understand what I’m missing? Thanks, and have a great weekend, -db

4 3

MonadFix wiki confusion
by Samuel Rødal 31 Oct '15

31 Oct '15

Hello, the MonadFix wiki at https://wiki.haskell.org/MonadFix has a statement that I feel is a bit misleading. In section "2.2 Lazy algorithm interleaved with effects", it claims that making the BTree data structure strict doesn't cause endless recursion. Well, that's true, but that's just because rep_x_sum returns a tuple containing the BTree and the summed values of the current subtree, and the tuple is lazily constructed - postponing the construction of the tree value. So highlighting the fact that the function still works when the BTree structure is made strict is kind of a red herring. Maybe the confusion could be avoided by removing the part about making BTree strict, or adding a note about the tuple still ensuring lazy construction? -- Samuel

3 4