September 2013 - Haskell-Cafe

ANN: th-desugar simplifies Template Haskell processing
by Richard Eisenberg 02 Sep '13

02 Sep '13

I've just uploaded my new th-desugar package, which enables easier processing of Template Haskell source syntax by desugaring it into a much simpler core language. The meaning of the code after desugaring is identical to before desugaring, but the syntax is much simpler. To wit, th-desugar simplifies out all of the following constructs: - guarded expressions in both functions and case statements - "where" declarations - do syntax - list/monad comprehensions - record creation / updates - as patterns - non-trivial patterns in a lambda expression - lambda-case - multi-way if - several more If you are writing a library that manipulates Template Haskell syntax, you may wish to consider if th-desugar will make your job easier by forcing you to consider fewer cases. The one source Haskell construct supported by Template Haskell but not supported by th-desguar is view patterns, mostly because these interact quite non-trivially with pattern binders. It's possible this hole will be closed in a future version. Enjoy! Richard

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Can I use String without "" in ghci?
by yi lu 02 Sep '13

02 Sep '13

I want to know if it is possible that I use strings without "". If I type *Prelude>foo bar* which actually I mean *Prelude>foo "bar"* However I don't want to type ""s. I have noticed if *bar* is predefined or it is a number, it can be used as arguments. But can other strings be used this way? Like in bash, we can use *ping 127.0.0.1* where *127.0.0.1* is an argument. If not, can *foo* be defined as a function so that it recognize arguments like *bar* as *"bar"*? Thanks, Yi Lu

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Re: [Haskell-cafe] function arithmetic?
by Christopher Howard 01 Sep '13

01 Sep '13

On 08/31/2013 09:27 PM, Charlie Paul wrote: > I believe that this is what you want: > http://www.haskell.org/haskellwiki/Num_instance_for_functions > > On Sat, Aug 31, 2013 at 10:01 PM, Christopher Howard > <christopher.howard(a)frigidcode.com> wrote: The author seemed to be subtly mocking the idea. It seemed to be suggesting that a Num instance for functions would imply the need for constant number functions, which leads to difficulties. But I don't see why one would have to take it that far. In any case, I just tried the NumInstances package from Hackage and it seems to work great.

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Proposal: Polymorphic typeclass and Records
by Wvv 01 Sep '13

01 Sep '13

Let we have data in one module as this: data Person = Person { personId :: Int, name :: String } data Address a = Address { personId :: Int, address :: String , way :: a} It was discussed a lot in topics "OverloadedRecordFields" This is an alternative: Let we have polymorphic typeclass: class Record{f} a b | a -> b where f :: a -> b so, compiler could create instances for our data as: instance Record{personId} Person Int where personId (Person x _) = x instance Record{personId} (Address a) Int where personId (Address x _ _) = x instance Record{way} (Address Int) Int where way (Address _ _ x) = x and we could use this: p:: Record {personId} r Int => r -> Int p = personId What do you think about this? -- View this message in context: http://haskell.1045720.n5.nabble.com/Proposal-Polymorphic-typeclass-and-Rec… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Compiler stops at SpecConstr optimization
by Daniel Díaz Casanueva 01 Sep '13

01 Sep '13

Hello. While hacking in one of my projects, one of my modules stopped to compile for apparently no reason. The compiler just freezes (like if it where in an infinite loop) while trying to compile that particular module. Since I had this problem I have been trying to reduce the problem as much as I could, and I came out with this small piece of code: > module Blah (foo) where > import Data.Vector (Vector) > import qualified Data.Vector as V > foo :: (a -> a) -> Vector a -> Vector a > foo f = V.fromList . V.foldl (\xs x -> f x : xs) [] The module I am importing comes from package vector-0.10.0.1. The compiler I am using is ghc-7.6.3. The problem arises when compiling using the following flags: ghc Blah.hs -O1 -fspec-constr If I compile it verbosely (with -v), I can see that the compiler stops in the SpecConstr optimization step. I have tried to reduce the -O1 flag to a particular set of flags, but I didn't manage yet to find the complete set of flags that -O1 translates to (optLevelFlags [1] seems to give not enough information). Since this problem can be OS-dependent, my system is Debian 7 and I didn't try yet to reproduce my problem in other systems (mainly because I don't have access to other systems at the moment). I am posting this here to, with the help of the Haskell Community, check first if this is indeed a bug and, in such case, to find out where that bug comes from (GHC, vector library, something else?). Then, if necessary, I would write a bug report where appropriate. Thank you, Daniel Díaz. -- References [1] - https://github.com/ghc/ghc/blob/ghc-7.6.3-release/compiler/main/DynFlags.hs…

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Hoogle vs Hayoo
by jabolopes＠google.com 01 Sep '13

01 Sep '13

Hi, I noticed Hayoo appears as a link in the toolbox of http://hackage.haskell.org and also that Hayoo seems to display better results than Hoogle. For example, if you search for 'PublicKey' in Hayoo, you will get several results from Hackage libraries, such as, 'crypto-pubkey' and 'crypto-api'. However, the same query in Hoogle displays no results. Is Hayoo the default Hackage search engine ? Is Hoogle deprecated ? What the status ? Thank you, Jose

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function arithmetic?
by Christopher Howard 01 Sep '13

01 Sep '13

Hi. I was just curious about something. In one of my math textbooks I see expressions like this f + g or (f + g)(a) where f and g are functions. What is meant is f(a) + g(a) Is there a way in Haskell you can make use of syntax like that (i.e., expressions like f + g and f * g to create a new function), perhaps by loading a module or something?

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