August 2013 - Haskell-Cafe

Categories with associated constraints?
by Conal Elliott 19 Aug '13

19 Aug '13

Has anyone given a go at a Category class and friends (including cartesian and closed) with associated constraints (presumably using the ConstraintKinds language extension)? I gave it a try a while back and wasn't able to keep the signatures from getting very complicated. Thanks, -- Conal

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Re: [Haskell-cafe] Errors in non-monadic code
by Donn Cave 19 Aug '13

19 Aug '13

jabolopes(a)google.com, MIME-Version: 1.0 Content-type: text/plain; charset=UTF-8 In-Reply-To: <CAKFCL4VfY-Dz3Xo9ZUZ_SmfRQ2nLGDLbovU=SuF1-sSnqVS8Hw(a)mail.gmail.com> References: <CAKFCL4VfY-Dz3Xo9ZUZ_SmfRQ2nLGDLbovU=SuF1-sSnqVS8Hw(a)mail.gmail.com> quoth Brandon Allbery, > Even in IO, exceptions should be reserved for truly exceptional conditions > (of the "program cannot safely continue" variety), not merely for error > checking when this can be described as a normal flow of evaluation. > Exceptions are not simply alternative flow of control, even in procedural > languages; they are *disruptions* of flow of control. I think that would be debatable, especially as applied to procedural languages in general. It's very common to use exceptions in routine flow of control in Python programs, if I remember right. Meanwhile, the Haskell library in my opinion somewhat weakens your case by making ordinary conditions into exceptions, for example end-of-file. In the end it's a large grey area. Ideally, what the program has to do, or whether it can even continue at all, is determined by the program using information reported by the function that encounters the error. If there's a good reason why exceptions should be used only when the program won't be able to continue, it must be that the language's exception handling facililty isn't very robust. Donn

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GHC and backwards compatibility
by Ketil Malde 19 Aug '13

19 Aug '13

I recently encountered the following problem: -------------------------------------------------------------------------------- $ cabal install Resolving dependencies... Configuring array-0.4.0.1... Building array-0.4.0.1... Preprocessing library array-0.4.0.1... Data/Array/IArray.hs:1:14: Unsupported extension: Trustworthy cabal: Error: some packages failed to install: array-0.4.0.1 failed during the building phase. The exception was: ExitFailure 1 asmeval-0 depends on array-0.4.0.1 which failed to install. binary-0.7.1.0 depends on array-0.4.0.1 which failed to install. biocore-0.3.1 depends on array-0.4.0.1 which failed to install. biofasta-0.0.3 depends on array-0.4.0.1 which failed to install. biopsl-0.4 depends on array-0.4.0.1 which failed to install. biosff-0.3.3 depends on array-0.4.0.1 which failed to install. bytestring-0.10.2.0 depends on array-0.4.0.1 which failed to install. containers-0.5.2.1 depends on array-0.4.0.1 which failed to install. deepseq-1.3.0.1 depends on array-0.4.0.1 which failed to install. hashable-1.2.0.10 depends on array-0.4.0.1 which failed to install. stringable-0.1.1 depends on array-0.4.0.1 which failed to install. system-filepath-0.4.7 depends on array-0.4.0.1 which failed to install. text-0.11.3.1 depends on array-0.4.0.1 which failed to install. unordered-containers-0.2.3.1 depends on array-0.4.0.1 which failed to install. -------------------------------------------------------------------------------- The installed GHC is 7.0.4, and yes, that's many major versions ago, but then again, it's only two years old, and from what I understand, it is shipped by some conservative Linux distributions. The problems here are that: a) the installation tries to install array, but current array requires the "Trustworthy" extension, which appeared in 7.2.1. Ideally, packages should try to be backwards compatible, e.g. by using conditional sections. b) the output isn't very helpful in tracking down the cause of this problem, it claims that all these packages depend on array-0.4.0.1, which is a lie. Somewhere, somehow, somethings depends on this (or at least a newer version), but I have no clue how to figure out which, except examining each of these packages and their dependencies manually. It is also possible that array shouldn't be upgraded like this, but then that needs to be made clear to cabal somehow, so that I don't spend ages trying to. Are there any solutions to this? Advice on how to proceed? -k -- If I haven't seen further, it is by standing in the footprints of giants

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Errors in non-monadic code
by jabolopes＠google.com 19 Aug '13

19 Aug '13

Hi, What is the proper way to implement a non-monadic function that checks whether a given value is correct and gives a proper error message otherwise ? What is the recommended option ? * Either String a check val | valid val = Right val | otherwise = Left errorMsg * Maybe String check val | valid val = Nothing | otherwise = Just errorMsg Cheers, Jose

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inv f g = f . g . f
by Christopher Done 19 Aug '13

19 Aug '13

Anyone ever needed this? Me and John Wiegley were discussing a decent name for it, John suggested inv as in involution. E.g. inv reverse (take 10) inv reverse (dropWhile isDigit) trim = inv reverse (dropWhile isSpace) . dropWhile isSpace That seems to be the only use-case I've ever come across. There's also this one: co f g = f g . g which means you can write trim = co (inv reverse) (dropWhile isSpace) but that's optimizing an ever rarer use-case.

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Database.postgreSQL.Simple - ambigious type
by Hartmut Pfarr 18 Aug '13

18 Aug '13

Hello, I've a problem connecting to my postgresql database. Can You help me fix the ambigious type signature? (The example is identical to the first 5-liner-example in the package documentation) http://hackage.haskell.org/packages/archive/postgresql-simple/0.3.5.0/doc/h… Kind regards Hartmut ------------------------------------------------------------------------ {-# LANGUAGE OverloadedStrings #-} import Database.PostgreSQL.Simple main = do conn <- connect defaultConnectInfo query conn "select 2 + 2" return () ------------------------------------------------------------------------ But this leads to error: ------------------------------------------------------------------------ Line 9: 1 error(s), 0 warning(s) Couldn't match expected type `IO a0' with actual type `q0 -> IO [r0]' In the return type of a call of `query' Probable cause: `query' is applied to too few arguments In a stmt of a 'do' block: query conn "select 2 + 2" In the expression: do { conn <- connect defaultConnectInfo; query conn "select 2 + 2"; return () } ------------------------------------------------------------------------ OK, I see, that a parameter q is missing. I change the source code to {-# LANGUAGE OverloadedStrings #-} import Database.PostgreSQL.Simple main = do conn <- connect defaultConnectInfo query conn "select 2 + 2" ( ) {- added ( ) here -} return () ------------------------------------------------------------------------ Now, I run into next error: ------------------------------------------------------------------------ Line 9: 1 error(s), 0 warning(s) No instance for (FromRow r0) arising from a use of `query' The type variable `r0' is ambiguous Possible fix: add a type signature that fixes these type variable(s) Note: there are several potential instances: instance (FromField a, FromField b) => FromRow (a, b) -- Defined in `Database.PostgreSQL.Simple.FromRow' instance (FromField a, FromField b, FromField c) => FromRow (a, b, c) -- Defined in `Database.PostgreSQL.Simple.FromRow' instance (FromField a, FromField b, FromField c, FromField d) => FromRow (a, b, c, d) -- Defined in `Database.PostgreSQL.Simple.FromRow' ...plus 10 others In a stmt of a 'do' block: query conn "select 2 + 2" () In the expression: do { conn <- connect defaultConnectInfo; query conn "select 2 + 2" (); return () } In an equation for `main': main = do { conn <- connect defaultConnectInfo; query conn "select 2 + 2" (); return () }

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MongoDB single insertions super slow
by Kyle Hanson 18 Aug '13

18 Aug '13

I am trying to write a simple benchmark of testing 1000 single inserts in mongodb, but doing single insertions is super slow. It takes nearly 40 seconds to perform these operations with the haskell mongodb driver, but under a second to perform the same number of operations of single inserts in python. Here are the two files I am working with: https://gist.github.com/hansonkd/6263648 I checked the BSON serialization, and writing it to a socket instead of mongodb driver, makes the script run in under a second so the problem seems to be with the mongodb driver. Any insights would be appreciated. -- Kyle Hanson

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ANN: processing-1.2.0.0. Graphic web applications with processing.js.
by Daniel Díaz Casanueva 18 Aug '13

18 Aug '13

Hello readers of Haskell-Cafe. Yesterday I uploaded a new version of the processing library [1]. == What is this library for? == The library is oriented to create graphic applications that run in a web browser, with no need of running a server. It provides the user with a set of functions and combinators that can be used to create an abstract piece code. This code is rendered to the processing language syntax. Once the code is uploaded, using processing.js [2], the output code is runnable from any browser with JavaScript support. To quickly give you a taste of the library, look at this animation, created with a simple recursive function: http://daniel-diaz.github.io/projects/processing/mill.html The code is here: https://github.com/Daniel-Diaz/processing/blob/master/examples/mill.hs == What's new? == After a couple of weeks working hard to have this library properly working, I think it have reached a point of reasonable usability. As a proof, I have written a simplified version of the Pac-Man game [3]. However, although I have implemented some code optimizations, it seems that the game still does not run smoothly in every web browser. I will definitely keep trying to improve the performance of the code generated by the library. I encourage you to try the library if you are interested, but keep in mind that it is still in an early stage, and may contain bugs, lack some important features, have an incomplete documentation or suffer from API changes. == References == [1] Library at Hackage - http://hackage.haskell.org/package/processing-1.2.0.0 [2] Processing.js - http://processingjs.org [3] Pac-Man game - http://daniel-diaz.github.io/projects/processing/pacman.html

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Alternative name for return
by J. Stutterheim 18 Aug '13

18 Aug '13

Dear Cafe, Suppose we now have the opportunity to change the name of the `return` function in Monad, what would be a "better" name for it? (for some definition of better) N.B. I am _not_ proposing that we actually change the name of `return`. I do currently have the opportunity to pick names for common functions in a non-Haskell related project, so I was wondering if there perhaps is a better name for `return`. - Jurriën

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Applicative is like an Arrow
by Thiago Negri 18 Aug '13

18 Aug '13

I just stumbled upon the Applicative term. Arrows are quite difficult for me to understand at the moment. I guess it needs time to digest. But, as I understand so far, Applicative and Arrows looks like the same thing. Please, enlight me.

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