October 2010 - Haskell-Cafe

Polyvariadic functions operating with a monoid
by Kevin Jardine 03 Oct '10

03 Oct '10

I had a situation where I had some related types that all had toString functions. Of course in Haskell, lists all have to be composed of values of exactly the same type, so instead of passing around lists of values with these related types, I created a polyvariadic function polyToString so that I could write: (polyToString value1 value2 value3 ... valueN) which would then become a list of strings: [toString value1, toString value2, ... , toString valueN] I finally figured out how to do this, but it was a bit harder to figure this out than I expected, and I was wondering if it might be possible to create a small utility library to help other developers do this. It seems to me that in the general case, we would be dealing with a Monoid rather than a list of strings. We could have a toMonoid function and then return polyToMonoid value1 value2 ... valueN = (toMonoid value1) `mappend` (toMonoid value2) 'mappend' ... (toMonoid valueN) So anyone who wanted to convert a bunch of values of different types to a Monoid could easily pass them around using polyToMonoid so long as they defined the appropriate toMonoid function. Basically, a generalised list. So I tried writing the following code but GHC said it had undecidable instances. Has this ever been done successfully? class Monoidable a where toMonoid :: Monoid r => a -> r polyToMonoid :: (Monoidable a, Monoid r) => a -> r polyToMonoid k = polyToMonoid' k mempty class PolyVariadic p where polyToMonoid' :: (Monoidable a, Monoid r) => a -> r -> p instance Monoid r => PolyVariadic r where polyToMonoid' k ss = (toMonoid k) `mappend` ss instance (Monoidable a, Monoid r) => PolyVariadic (a -> r) where polyToMonoid' k ss = (\a -> polyToMonoid' k (toMonoid a) `mappend` ss)

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Non-existing types in existential quantification?
by Henning Thielemann 03 Oct '10

03 Oct '10

Given the following code, that is accepted by GHC: > data Exist = forall a. Exist a > > exist :: Exist > exist = Exist undefined What type has the 'undefined' ? So far I assumed that at runtime all objects have a concrete type. This seems not to be true. I can also write > data ExistList = forall a. ExistList [a] > > exist :: ExistList > exist = ExistList [] where I do not need an 'undefined'.

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Haskell web development entries on the Wiki
by Christopher Done 03 Oct '10

03 Oct '10

Decided to move this to a separate thread. I went ahead and refactored the wiki entries: http://www.haskell.org/haskellwiki/Web Now we have the following Web/ sections: Servers Frameworks Interfaces to frameworks Databases and Persistence Libraries Testing and Verification Content Management and Forums and Discussion Literature (research, talks and blogs) Existing Haskell web applications Ongoing projects and ideas And Web/Libraries has the following sections: URLs and Routing Templating Sessions and Authentication CGI Forms JavaScript and AJAX CSS I will add all these pages to the Web category, but not yet, I want to get rid of this page: http://www.haskell.org/haskellwiki/?title=Applications_and_libraries/Web_pr… I've nearly finished moving all of it to organised sections in this new hierarchy. The advantage of doing this is that I have to manually check each thing listed and I discover that quite a few are dead links and no longer maintained. Ideally this page http://www.haskell.org/haskellwiki/Web/Frameworks would have a list of all actually active web projects, and those would be listed at the top, or perhaps there should be another page Web/Frameworks/Active or whatnot. Anyone care to help take this page apart Applications and libraries/Web programming and move it to reasonable pages? Thoughts?

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I still cannot seem to get a GUI working under Windows.
by caseyh＠istar.ca 03 Oct '10

03 Oct '10

I still cannot seem to get a GUI working under Windows. For Haskell GUI's is Ubuntu easier to setup. If so, we're losing people if Haskell GUI's are so hard to get working under Windows.

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Re: [Haskell-cafe] Coding conventions for Haskell?
by Andrew Coppin 03 Oct '10

03 Oct '10

On 29/09/2010 02:18 PM, Henning Thielemann wrote: > Andrew Coppin wrote: > >> Tastes do indeed vary. To me, both of these are incorrect, and the >> correct way is >> >> data Foo a b = >> Foo a | >> Bar b | >> Foobar a b >> deriving (Eq, Ord) > > The truth is: Given the separator style of constructor definition, > there is no correct way to format those declarations. :-) The correct > way would be to allow terminator style. Well, yes, there is that. (And this isn't the only place in the syntax where it applies either. Tried editing export lists lately? Or Cabal module lists?)

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Inverse of HaskellDB
by Jonathan Geddes 02 Oct '10

02 Oct '10

Cafe, HaskellDB takes a database schema and produces Haskell data structures (plus some other query-related stuff for its EDSL query language). What I'm looking for is the inverse of this functionality. I want to create tables based on a Haskell data structure with a few simple rules. These rules include: if a field is not of the form `Maybe a' then it can't be nullable in the database. If a field is not a primitive (in the database) then it is actually stored in another table and a reference id is stored in the table. Tables are produced recursively, unless they already exist, etc. The HaskellDB approach is great for interfacing with existing tables, but in my case I already have data structures and now I would like a quick way to create tables to persist them. Does such a thing exist? If not, would you find it useful? I may take this up as a side project if it does not already exist and others would find it useful. Thanks, --Jonathan

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The Haskell Implementors Workshop
by Don Stewart 02 Oct '10

02 Oct '10

Slides and videos from today's Haskell Implementors Workshop, in Baltimore. http://haskell.org/haskellwiki/HaskellImplementorsWorkshop/2010#Programme Enjoy. -- Don

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Haskell list on Twitter?
by Magnus Therning 01 Oct '10

01 Oct '10

Now that there are lists/groups on Twitter (http://mashable.com/2009/11/02/twitter-lists-guide/) maybe one should be created one for Haskellers? I would do it, but it seems I can't be on my own list :-) So unless I've missed something I'd have to create a dummy user just for this purpose. Hence I'm asking first :-) Cheers, M -- Magnus Therning (OpenPGP: 0xAB4DFBA4) magnus＠therning．org Jabber: magnus＠therning．org http://therning.org/magnus identi.ca|twitter: magthe

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Coding conventions for Haskell?
by Petr Pudlak 01 Oct '10

01 Oct '10

Hi, sometimes I have doubts how to structure my Haskell code - where to break lines, how much to indent, how to name functions and variables etc. Are there any suggested/recommended coding conventions? I searched a bit and I found a few articles and discussions: - Good Haskell coding standards from Stackoverflow http://stackoverflow.com/questions/1983047/good-haskell-coding-standards - Programming guidelines from Haskellwiki http://www.haskell.org/haskellwiki/Programming_guidelines - How to read Haskell from Haskellwiki (this is actually about reading, not composing code, but still worth reading) http://www.haskell.org/haskellwiki/How_to_read_Haskell - Good Haskell Style http://urchin.earth.li/~ian/style/haskell.html but they usually address only a few specifics. I was looking for something more complete and comprehensive. Perhaps something like Java has [1]. Any suggestions? Best regards, Petr [1] http://www.oracle.com/technetwork/java/codeconv-138413.html

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record update
by Jonathan Geddes 01 Oct '10

01 Oct '10

I know that record updates is a topic that has become a bit of a dead horse, but here I go anyway: I find that most of the record updates I read and write take the form >someUpdate :: MyRecord -> MyRecord >someUpdate myRecord = myRecord > { field1 = f $ field1 myRecord > , field2 = g $ field2 myRecord > , field3 = h $ filed3 myRecord > } I find myself wishing I could write something more like >someUpdate :: MyRecord -> MyRecord >someUpdate myRecord = myRecord > { field1 => f > , field2 => g > , field3 => h > } with equivalent semantics. Here => reads "is transformed by". Operator = could still be used for assignment as in current record updates. The best part about such an extension, in my opinion, is that it would open the door for anonymous lambda record updates. Something like: >someUpdate :: MyRecord -> MyRecord >someUpdate = \{field1 => f, field2 => g, field3 => h} again, with the same semantics. This becomes possible because you no longer need to refer to the record within the {} part of the update. This would be useful, for example, in the State monad. We could write: >someStateTransform :: State MyRecord () >someStateTransform = do > modify $ \{field1 => (++"!")} > ... where currently we see code like >someStateTransform :: State MyRecord () >someStateTransform = do > modify $ \record->record{field1 = (++"!") $ field1 record} > ... which repeats the record name 3 times and the field name twice. The repetition just feels out of place next to all the other terse, readable Haskell code in the program. So what do my fellow haskellers think? Is this idea worth writing up a proposal for? Alternatively, can you offer me some advice on writing code in Haskell 2010 that avoids the ugly, repetitive style of record update? --Jonathan

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