Re: [Haskell-cafe] MonadBaseControl IO instance for conduits ?
Aleksey Uymanov s9gf4...@gmail.com writes: Is it posible to create instance of MonadBaseControl IO (ConduitM i o m) ? No, it is not, for approximately the same reason that you cannot create one for ContT (or any form of continuation). -- John Wiegley FP Complete Haskell tools, training and consulting http://fpcomplete.com johnw on #haskell/irc.freenode.net ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Lifting IO actions into Applicatives
Daniil Frumin difru...@gmail.com writes: Isn't it the case that there could be more than one natural transformation between functors? Yes, I imagine there would have to be some newtype wrappers to distinguish in those cases. -- John Wiegley FP Complete Haskell tools, training and consulting http://fpcomplete.com johnw on #haskell/irc.freenode.net ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Lifting IO actions into Applicatives
Yitzchak Gale g...@sefer.org writes:
In fact, it even makes sense to define it as FunctorIO, with the only laws
being that liftIO commutes with fmap and preserves id, i.e., that it is a
natural transformation. (Those laws are also needed for ApplicativeIO and
MonadIO.)
Given that we are moving toward Applicative (and thus Functor) as a superclass
of Monad, why not just solve the MonadIO problem and similar type classes with
natural transformations? It requires 3 extensions, but these are extensions I
believe should become part of Haskell anyway:
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE RankNTypes #-}
module NatTrans where
import Control.Monad.IO.Class
import Control.Monad.Trans.Maybe
class (Functor s, Functor t) = NatTrans s t where
nmap :: forall a. s a - t a
-- Such that: nmap . fmap f = fmap f . nmap
-- In 7.10, this Functor constraint becomes redundant
instance (Functor m, MonadIO m) = NatTrans IO m where
nmap = liftIO
main :: IO ()
main = void $ runMaybeT $ nmap $ print (10 :: Int)
Now if I have a functor of one kind and need another, I reach for nmap in the
same way that I reach for fmap to transform the mapped type.
--
John Wiegley
FP Complete Haskell tools, training and consulting
http://fpcomplete.com johnw on #haskell/irc.freenode.net
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Re: [Haskell-cafe] Creating a local Hoogle ...
aditya siram aditya.si...@gmail.com writes: Combining 4263 databases hoogle: embroidery.hoo: openFile: resource exhausted (Too many open files) Any help is appreciated. Thanks! I created the 'rehoo' utility to solve this very problem. Change directory to where your .hoo files are, and run: rehoo -j4 -c64 . Where 4 is for a 4-core machine. -- John Wiegley FP Complete Haskell tools, training and consulting http://fpcomplete.com johnw on #haskell/irc.freenode.net ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] This is a mail system test, please ignore
Testing the new e-mail services at haskell.org. -- John Wiegley FP Complete Haskell tools, training and consulting http://fpcomplete.com johnw on #haskell/irc.freenode.net ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Please excuse brief service disruption
The e-mail services at haskell.org were switched from exim4 to postfix tonight, please excuse the service disruption. For about one hour e-mails were not being accepted to the mailing lists. All should be resolved now. -- John Wiegley FP Complete Haskell tools, training and consulting http://fpcomplete.com johnw on #haskell/irc.freenode.net ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Proposal: New syntax for Haskell
Niklas Hambüchen m...@nh2.me writes: Code written in cucumber syntax is concise and easy to read concise |kənˈsīs|, adj. giving a lot of information clearly and in a few words; brief but comprehensive. Compare: Scenario: Defining the function foldl Given I want do define foldl Which has the type (in brackets) a to b to a (end of brackets), to a, to list of b, to a And my arguments are called f, acc, and l When l is empty Then the result better be acc Otherwise l is x cons xs Then the result should be foldl f (in brackets) f acc x (end of brackets) xs To: foldl :: (a - b - a) - a - [b] - a foldl f z [] = z foldl f z (x:xs) = foldl f (f z x) xs How is that more concise or preferable? -- John Wiegley FP Complete Haskell tools, training and consulting http://fpcomplete.com johnw on #haskell/irc.freenode.net ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] definition of the term combinator
Jason Dagit dag...@gmail.com writes: Where can I find a formal and precise definition of the term combinator, A function that uses nothing but its arguments. as a term used by the Haskell community to describe something? I find that Haskellers often use combinator to mean a function that makes new functions out of other functions, which it can often do as a pure combinator, but isn't always a combinator per se. -- John Wiegley FP Complete Haskell tools, training and consulting http://fpcomplete.com johnw on #haskell/irc.freenode.net ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] inv f g = f . g . f
Dan Burton danburton.em...@gmail.com writes: under reversed (take 10) ['a'.. 'z'] qrstuvwxyz Excellent, thanks! -- John Wiegley FP Complete Haskell tools, training and consulting http://fpcomplete.com johnw on #haskell/irc.freenode.net ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] ANNOUNCE: tasty, a new testing framework
Roman Cheplyaka r...@ro-che.info writes: I am pleased to announce the first release of tasty, a new testing framework for Haskell. It is meant to be a successor to test-framework (which is unmaintained). It would be nice to see a comparison of the various test frameworks and why one might select one over another. I use hspec currently (which also integrates with HUnit, QuickCheck, etc.), and couldn't tell at a glance what tasty might offer. And I particularly dislike writing tests inside of a gigantic list; I much prefer the monadic style of hspec. -- John Wiegley FP Complete Haskell tools, training and consulting http://fpcomplete.com johnw on #haskell/irc.freenode.net ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] ScopedTypeVariables
Evan Laforge qdun...@gmail.com writes: Would it make sense to split it into a separate extension like TypesOnArguments so I can more accurately express my deviation from haskell2010 orthodoxy? Or is there some deeper tie between scoped type variables and annotations on arguments? I've also wondered why I have to enable ScopedTypeVariables in those cases -- when I'm not turning it on to scope any type variables, but just to make annotations possible in more places. -- John Wiegley FP Complete Haskell tools, training and consulting http://fpcomplete.com johnw on #haskell/irc.freenode.net ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] ANNOUNCE: module-management-0.9.3 - clean import lists, split and merge modules
David Fox d...@seereason.com writes: Cliff Beshers wrote a CLI for this, I will add it as a cabal executable in the next version. Oh, also, I was unable to build the library using GHC 7.4.2. It looks like it still depends on the old Exception stuff that used to be Prelude? -- John Wiegley FP Complete Haskell tools, training and consulting http://fpcomplete.com johnw on #haskell/irc.freenode.net ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] ANNOUNCE: module-management-0.9.3 - clean import lists, split and merge modules
David Fox d...@seereason.com writes: I am pleased to announce the first release of module-management, a package for cleaning import lists, and splitting and merging modules. You can see a description at the top of the documentation for Language.Haskell.Modules (once it appears) here: How about building an executable along with the library called cleanImports, so that I can use it from the command-line. Otherwise, every who wants to use your library in this way will be writing pretty much the exact same code. cleanImports is something I've been wanting, just hadn't gotten around to writing it yet. Thanks! -- John Wiegley FP Complete Haskell tools, training and consulting http://fpcomplete.com johnw on #haskell/irc.freenode.net ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Nightly builds of GHC HEAD for OS X 10.8
John Wiegley jo...@fpcomplete.com writes: Since mid-January, I’ve been running nightly builds of GHC on my Mac Pro for 10.8.x, 64-bit. I’ve decided to make these results publically downloadable here: http://ghc.newartisans.com. Just a note: I'm now including builds and logs for Ubuntu 12.04.x LTS 64-bit as well, in addition to OS X 10.8.x. If anyone has another platform they'd like to add, and can give me SSH access to an account on their machine, the build needs about 10G of disk space and eight hours a day of full CPU access (on average). It can run in whatever time range works best for you. Let me know by e-mail if you're interested in contributing to these nightlies. As a next step, I plan to build source tarballs as well, so that the binaries can be used forensically to allow bisecting among the builds for a breakage. -- John Wiegley FP Complete Haskell tools, training and consulting http://fpcomplete.com johnw on #haskell/irc.freenode.net ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Nightly builds of GHC HEAD for OS X 10.8
Since mid-January, I’ve been running nightly builds of GHC on my Mac Pro for 10.8.x, 64-bit. I’ve decided to make these results publically downloadable here: http://ghc.newartisans.com. The installer tarballs are in dist, while the fulltest and nofib logs are in logs. According to Jenkins this build takes 8h15m minutes, so I figured this might save others some CPU heat. -- John Wiegley FP Complete Haskell tools, training and consulting http://fpcomplete.com johnw on #haskell/irc.freenode.net ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] GSoC Project Proposal: Markdown support for Haddock
Johan Tibell johan.tib...@gmail.com writes: I suggest that we implement an alternative haddock syntax that's a superset of Markdown. Definite +1 from me too. -- John Wiegley FP Complete Haskell tools, training and consulting http://fpcomplete.com johnw on #haskell/irc.freenode.net ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Threadscope 0.2.2 goes in segmentation fault on Mac Os X 10.8.3
Alfredo Di Napoli alfredo.dinap...@gmail.com writes: I know it's a bit difficult to debug this way, I can try debugging with gdb if it can help. Yes, can you show us a backtrace from gdb, and also look in your CrashReports log folder to see if it gives a bit more information on the state of the process at the time it died? Thanks, -- John Wiegley FP Complete Haskell tools, training and consulting http://fpcomplete.com johnw on #haskell/irc.freenode.net ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] ANN: lazy-csv - the fastest and most space-efficient parser for CSV
Malcolm Wallace malcolm.wall...@me.com writes: Simple answer - I have never heard of cassava, and suspect it did not exist when I first did the benchmarking. I'd be happy to re-do my performance comparison, including cassava and any other recent-ish CSV libraries, if I can find them. I would be very interested in those results, Malcolm. Thanks, -- John Wiegley FP Complete Haskell tools, training and consulting http://fpcomplete.com johnw on #haskell/irc.freenode.net ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Use forM_ with Maybe, it's Foldable!
Felipe Almeida Lessa felipe.le...@gmail.com writes: A few days ago I decided to hoogle the type of whenJust [2] and what I discovered is that import Data.Foldable (forM_) whenJust = forM_ You can also use for_, if you want to use Applicative instead of Monad. -- John Wiegley FP Complete Haskell tools, training and consulting http://fpcomplete.com johnw on #haskell/irc.freenode.net ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] ANN: monad-bool 0.1
Petr P petr@gmail.com writes: Don't take it so hard. Trying to reinvent something is always a great exercise and makes you really understand the problem. And it can have interesting results too. One of my university professors once heard about some concept, but didn't know the details. He tried to derive the concept himself, and he actually invented something different, new and very useful. Thank you very much to everyone for the encouragement, and especially to Edward and Shachaf for the education on #haskell. It will be hard to unlearn a lesson like this one. :) And I will certainly endeavor to make the most valuable mistakes I can from here on, with the help of such a gracious community. :) Yours, -- John Wiegley FP Complete Haskell tools, training and consulting http://fpcomplete.com johnw on #haskell/irc.freenode.net ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] ANN: monad-bool 0.1
monad-bool implements a pair of Boolean monoids and monads, to support short-circuiting, value-returning computations similar to what Python and Ruby offer with their native and || operators. For example, in Python you might see this: x = [1,2,3,0] print x[1] || x[3] -- prints 2 With this library, you can now mirror such code in Haskell: let x = [1,2,3,0] print $ (x !! 1) ||? (x !! 3) -- prints Success 2 Booleanness is based on each type having an instance of the 'Control.Conditional.ToBool' type, for which only the basic types are covered (Bool, Int, Integer, Maybe a, Either a b, [a], Attempt a). If you wish to define a truth value for your own types, simply provide an instance for ToBool: instance ToBool MyType where toBool = ... The And/Or monoids use the Attempt library so that the actual type of the successful results depends on case analysis. It could be a list, a Maybe, an Either, or an exception in the IO Monad. The monad variants, AndM, AndMT, OrM and OrMT provide short-circuiting behavior in a Monad, which returns the last value returned before truth was determined. Here are two examples: Use 'onlyIf' with AndM and AndMT to guard later statements, which are only evaluated if every preceding 'onlyIf' evaluates to True. For example: foo :: AndM Int foo = do onlyIf (True == True) return 100 onlyIf (True == True) return 150 onlyIf (True == False) return 200 When run with `evalAndM foo (-1)` (where (-1) provides a default value), 'foo' returns 150. Use 'endIf' with OrM and OrMT to chain statements, which are only executed if every preceding 'endIf' evaluated to False. For example: bar :: OrM Int bar = do endIf (True == False) return 100 endIf (True == False) return 150 endIf (True == True) return 200 When run with `evalOrM bar (-1)` (where (-1) again provides a default value), 'bar' returns 150. And please, somebody let me know if this has already been done. A search for likely candidates did not turn up anything obvious in Hoogle, but as my knowledge of the whole of Hackage is minimal, I would appreciate any wiser minds that can inform me. Thank you, -- John Wiegley FP Complete Haskell tools, training and consulting http://fpcomplete.com johnw on #haskell/irc.freenode.net ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] [Haskell] ANN: monad-bool 0.1
Henning Thielemann lemm...@henning-thielemann.de writes: Does the And monad fulfill the monad laws? In a proper monad an interim (return x) (without a '-') is a no-op. You are very right. I will make the necessary changes. -- John Wiegley FP Complete Haskell tools, training and consulting http://fpcomplete.com johnw on #haskell/irc.freenode.net ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] ANN: monad-bool 0.1
Conrad Parker con...@metadecks.org writes: these sound powerful, but how would I do something esoteric like if/elseIf/endIf ? Can you show me an example of what you'd like to express? -- John Wiegley FP Complete Haskell tools, training and consulting http://fpcomplete.com johnw on #haskell/irc.freenode.net ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] ANN: monad-bool 0.1
Roman Cheplyaka r...@ro-che.info writes: - what do you need unsafeCoerce for? Ok, after much discussion with edwardk and shachaf, I came to realize a few things: 1. The functionality of my two monads 'AndM' and 'OrR' can be boiled down to two helper functions inside EitherT: returnIf :: Monad m = Bool - e - EitherT e m () returnIf p a = if p then left a else right () returnUnless :: Monad m = Bool - e - EitherT e m () returnUnless p = returnIf (not p) These let you short-circuit monadic computations, returning the short-circuiting value as a Left; or as a Right if it reaches the end. 2. The 'shortcircuit' library already provides short-circuiting variants of and || that work just like Python's and Ruby's. In fact, I think I'll talk to aristid about merging my Monoid definitions into that library. Here are the updated docs: http://ftp.newartisans.com/pub/monoid-bool/Data-Monoid-Bool.html I'll ask Ross Paterson to deprecate monad-bool. And in future, I'll seek review here first before uploading. -- John Wiegley FP Complete Haskell tools, training and consulting http://fpcomplete.com johnw on #haskell/irc.freenode.net ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] ANN: monad-bool 0.1
Ertugrul Söylemez e...@ertes.de writes: There is a good reason why Haskell's type system would never have allowed to write this library. I recommend the author to try again without unsafeCoerce. It won't work. You are right, and in fact what I wanted to do cannot be done. It requires distinguishing the final value as either a Left (result from short-circuiting) or a Right (final value). And EitherT can already do that, it just lacks a convenience function to make it easier (i.e, not having to write lots of nested if statements). -- John Wiegley FP Complete Haskell tools, training and consulting http://fpcomplete.com johnw on #haskell/irc.freenode.net ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] ANN: monad-bool 0.1
John Wiegley jo...@fpcomplete.com writes: And EitherT can already do that, it just lacks a convenience function to make it easier (i.e, not having to write lots of nested if statements). Never mind, when/unless + left work just fine for this. -- John Wiegley FP Complete Haskell tools, training and consulting http://fpcomplete.com johnw on #haskell/irc.freenode.net ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] ANN: monad-bool 0.1
John Wiegley jo...@fpcomplete.com writes: Never mind, when/unless + left work just fine for this. You know, it's been a humorous day. First ekmett showed that I can't make a sane Monad instance for AndM or OrM. Then I discovered I can't make a reasonable Monoid (no mempty, given only 'toBool'), so I dropped down to a Semigroup. Further, my combinators for EitherT can be implemented using just 'when' and 'left' to provide the short-circuiting. Already I had very little code left, until he showed me the Applicative instance for Either, plus a little trick: Right 1 * Right 2 * Left 2 * Right 5 Left 2 -- same functionality as my And semigroup let Left x | y = y; x | _ = x in Left 1 | Right 2 | Right 3 Right 2-- same functionality as my Or semigroup And poof, all my code just disappeared... -- John Wiegley FP Complete Haskell tools, training and consulting http://fpcomplete.com johnw on #haskell/irc.freenode.net ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Is there a tool like ri from ruby?
Tikhon Jelvis tik...@jelv.is writes: Have you tried Hoogle? I know you can install it locally and use it from GHCi or Emacs. I'm not familiar with ri, but from your description I think a local Hoogle would serve the same purpose with the added benefit of being able to search by types. Here's the wiki page about it: http://www.haskell.org/haskellwiki/Hoogle See also: http://newartisans.com/2012/09/running-a-fully-local-hoogle/ With this setup I can use :doc head in my ghci sessions, and see docs very similar to what ri would show. -- John Wiegley FP Complete Haskell tools, training and consulting http://fpcomplete.com johnw on #haskell/irc.freenode.net ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Category Theory and Haskell
Gytis Žilinskas gytis.zilins...@gmail.com writes: How difficult would it be to study category theory and simultaneously come up with Haskell examples of various results that it presents? There are some aspects of CT that you will not be able to express in Haskell easily (try encoding the forgetful functor, for instance). But there are other things which translate quite nicely. As one example, I've been using Haskell to explore the connection between adjunctions and monads, and have found it quite rewarding. My efforts so far are here: https://github.com/jwiegley/posts/blob/master/Adjunctions%20in%20Haskell/Adjunction.hs What I found most compelling from that code is I only need a minimal definition of the Prod ⊣ Hom adjunction, (ε and η -- and their definitions are trivial), I get all the behavior of the well-known State monad, whose typical implementation via = is rather tricky. Doing this exploration in Haskell -- with the aid of the type-checker -- made certain connections clear that were difficult to see in the abstract. And it was plenty of fun besides. :) Happy brain spelunking, -- John Wiegley FP Complete Haskell tools, training and consulting http://fpcomplete.com johnw on #haskell/irc.freenode.net ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Instead of Haskell running on the JVM is there a way for Haskell to call a JVM language ...
KC kc1...@gmail.com writes: Instead of Haskell running on the JVM is there a way for Haskell to call a JVM language (or generate bytecode) to access the Java class libraries when needed? Or Is there a way for a JVM language or bytecode to call Haskell when needed? I'd be very interested to know the answer to this as well. Please let me know what you discover. John ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] foldr (.) id
Greg Fitzgerald gari...@gmail.com writes: I've recently found myself using the expression: foldr (.) id to compose a list (or Foldable) of functions. You want the Endo monoid: ghci appEndo (Endo (+ 10) Endo (+ 20)) $ 3 33 John ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] foldr (.) id
Thiago Negri evoh...@gmail.com writes: Can you please show some examples where it might be useful? I miss the point. I guess if he already has a list of functions, Endo won't help. Endo just lets you treat functions as monoids, so you can foldMap, etc. In that case, foldr (.) id is pretty idiomatic, and Google turns up several uses of it. John ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Is Hackage down?
C K Kashyap ckkash...@gmail.com writes: Is it just me or is Hackage indeed been going down more frequently of late? It's not just you. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Over general types are too easy to make.
timothyho...@seznam.cz writes:
data BadFoo =
BadBar{
badFoo::Int} |
BadFrog{
badFrog::String,
badChicken::Int}
This is fine, until we want to write a function that acts on Frogs but not
on Bars. The best we can do is throw a runtime error when passed a Bar and
not a Foo:
You can use wrapper types to solve this:
data BadBarType = BadBarType BadFoo
data BadFrogType = BadFrogType BadFoo
Now you can have:
deBadFrog :: BadFrogType - String
And call it as:
deBadFrog $ BadFrogType (BadFrog { badFrog = Hey, badChicken = 1})
Needless to say, you will have to create helper functions for creating Bars
and Frogs, and not allow your BadBar or BadFrog value constructors to be
visible outside your module.
John
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