June 2009 - Haskell-Cafe

Re: [Haskell-cafe] Missing a "Deriving"?
by michael rice 01 Jun '09

01 Jun '09

I didn't know I could do that. Works fine. Output below. Thanks! This is some pretty neat stuff, and I've only scratched the surface. Michael =================== [michael@localhost ~]$ ghci GHCi, version 6.10.1: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. Prelude> :l graph5 [1 of 1] Compiling Main ( graph5.hs, interpreted ) graph5.hs:37:9: Warning: No explicit method nor default method for `mzero' In the instance declaration for `MonadPlus Failable' Ok, modules loaded: Main. *Main> let g = Graph [(1,'a'),(2,'b'),(3,'c'),(4,'d')] [(1,2,'p'),(2,3,'q'),(1,4,'r'),(4,3,'s')] *Main> searchAll g 1 3 :: Failable [Int] Success [1,2,3] *Main> searchAll g 3 1 :: Failable [Int] Fail "no path" *Main> searchAll g 1 3 :: Maybe [Int] Just [1,2,3] *Main> searchAll g 3 1 :: Maybe [Int] Nothing *Main> searchAll g 1 3 :: [[Int]] [[1,2,3],[1,4,3]] *Main> searchAll g 3 1 :: [[Int]] [] *Main> --- On Mon, 6/1/09, Ross Mellgren <rmm-haskell(a)z.odi.ac> wrote: From: Ross Mellgren <rmm-haskell(a)z.odi.ac> Subject: Re: [Haskell-cafe] Missing a "Deriving"? To: "michael rice" <nowgate(a)yahoo.com> Cc: "haskell-cafe Cafe" <haskell-cafe(a)haskell.org> Date: Monday, June 1, 2009, 9:43 PM Oh I wasn't clear -- you need multiple instance declarations for a given type (Failable, for example), one for each type class you're implementing. That is, instance Monad Failable where return = ... ... instance MonadPlus Failable where mplus = ... ... -Ross On Jun 1, 2009, at 9:40 PM, michael rice wrote: Hi Ross, I thought of that, but return, fail, and >>= became "not visible" when I changed the instance declaration from Monad to MonadPlus.. Can Failable be in two instance declarations, one for Monad (giving it return, fail, and >>=) and one for MonadPlus (giving it mplus)? Michael --- On Mon, 6/1/09, Ross Mellgren <rmm-haskell(a)z.odi.ac> wrote: From: Ross Mellgren <rmm-haskell(a)z.odi.ac> Subject: Re: [Haskell-cafe] Missing a "Deriving"? To: "michael rice" <nowgate(a)yahoo.com> Cc: "haskell-cafe Cafe" <haskell-cafe(a)haskell.org> Date: Monday, June 1, 2009, 9:33 PM mplus is a method of class MonadPlus, so you need to write it in a separate instance from the one for Monad, e.g. instance MonadPlus Failable where mplus = ... -Ross On Jun 1, 2009, at 9:28 PM, michael rice wrote: Still stumped. Maybe and [] are in the same MonadPlus monad, but how do I make monad Failable understand mplus? I'm now getting this error upon loading: Prelude> :l graph5 [1 of 1] Compiling Main ( graph5.hs, interpreted ) graph5.hs:36:4: `mplus' is not a (visible) method of class `Monad' Failed, modules loaded: none. Prelude> Complete code follows. Michael ========================= import Monad data Failable a = Success a | Fail String deriving (Show) data Graph v e = Graph [(Int,v)] [(Int,Int,e)] {- class Computation c where success :: a -> c a failure :: String -> c a augment :: c a -> (a -> c b) -> c b combine :: c a -> c a -> c a instance Computation Maybe where success = Just failure = const Nothing augment (Just x) f = f x augment Nothing _ = Nothing combine Nothing y = y combine x _ = x instance Computation Failable where success = Success failure = Fail augment (Success x) f = f x augment (Fail s) _ = Fail s combine (Fail _) y = y combine x _ = x -} instance Monad Failable where return = Success fail = Fail (>>=) (Success x) f = f x (>>=) (Fail s) _ = Fail s mplus (Fail _) y = y mplus x _ = x {- instance Computation [] where success a = [a] failure = const [] augment l f = concat (map f l) combine = (++) searchAll g@(Graph vl el) src dst | src == dst = success [src] | otherwise = search' el where search' [] = failure "no path" search' ((u,v,_):es) | src == u = (searchAll g v dst `augment` (success . (u:))) `combine` search' es | otherwise = search' es -} searchAll g@(Graph vl el) src dst | src == dst = return [src] | otherwise = search' el where search' [] = fail "no path" search' ((u,v,_):es) | src == u = (searchAll g v dst >>= (return . (u:))) `mplus` search' es | otherwise = search' es -----Inline Attachment Follows----- _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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Re: [Haskell-cafe] Missing a "Deriving"?
by michael rice 01 Jun '09

01 Jun '09

Hi Ross, I thought of that, but return, fail, and >>= became "not visible" when I changed the instance declaration from Monad to MonadPlus.. Can Failable be in two instance declarations, one for Monad (giving it return, fail, and >>=) and one for MonadPlus (giving it mplus)? Michael --- On Mon, 6/1/09, Ross Mellgren <rmm-haskell(a)z.odi.ac> wrote: From: Ross Mellgren <rmm-haskell(a)z.odi.ac> Subject: Re: [Haskell-cafe] Missing a "Deriving"? To: "michael rice" <nowgate(a)yahoo.com> Cc: "haskell-cafe Cafe" <haskell-cafe(a)haskell.org> Date: Monday, June 1, 2009, 9:33 PM mplus is a method of class MonadPlus, so you need to write it in a separate instance from the one for Monad, e.g. instance MonadPlus Failable where mplus = ... -Ross On Jun 1, 2009, at 9:28 PM, michael rice wrote: Still stumped. Maybe and [] are in the same MonadPlus monad, but how do I make monad Failable understand mplus? I'm now getting this error upon loading: Prelude> :l graph5 [1 of 1] Compiling Main ( graph5.hs, interpreted ) graph5.hs:36:4: `mplus' is not a (visible) method of class `Monad' Failed, modules loaded: none. Prelude> Complete code follows. Michael ========================= import Monad data Failable a = Success a | Fail String deriving (Show) data Graph v e = Graph [(Int,v)] [(Int,Int,e)] {- class Computation c where success :: a -> c a failure :: String -> c a augment :: c a -> (a -> c b) -> c b combine :: c a -> c a -> c a instance Computation Maybe where success = Just failure = const Nothing augment (Just x) f = f x augment Nothing _ = Nothing combine Nothing y = y combine x _ = x instance Computation Failable where success = Success failure = Fail augment (Success x) f = f x augment (Fail s) _ = Fail s combine (Fail _) y = y combine x _ = x -} instance Monad Failable where return = Success fail = Fail (>>=) (Success x) f = f x (>>=) (Fail s) _ = Fail s mplus (Fail _) y = y mplus x _ = x {- instance Computation [] where success a = [a] failure = const [] augment l f = concat (map f l) combine = (++) searchAll g@(Graph vl el) src dst | src == dst = success [src] | otherwise = search' el where search' [] = failure "no path" search' ((u,v,_):es) | src == u = (searchAll g v dst `augment` (success . (u:))) `combine` search' es | otherwise = search' es -} searchAll g@(Graph vl el) src dst | src == dst = return [src] | otherwise = search' el where search' [] = fail "no path" search' ((u,v,_):es) | src == u = (searchAll g v dst >>= (return . (u:))) `mplus` search' es | otherwise = search' es -----Inline Attachment Follows----- _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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Re: [Haskell-cafe] Missing a "Deriving"?
by michael rice 01 Jun '09

01 Jun '09

Still stumped. Maybe and [] are in the same MonadPlus monad, but how do I make monad Failable understand mplus? I'm now getting this error upon loading: Prelude> :l graph5 [1 of 1] Compiling Main ( graph5.hs, interpreted ) graph5.hs:36:4: `mplus' is not a (visible) method of class `Monad' Failed, modules loaded: none. Prelude> Complete code follows. Michael ========================= import Monad data Failable a = Success a | Fail String deriving (Show) data Graph v e = Graph [(Int,v)] [(Int,Int,e)] {- class Computation c where success :: a -> c a failure :: String -> c a augment :: c a -> (a -> c b) -> c b combine :: c a -> c a -> c a instance Computation Maybe where success = Just failure = const Nothing augment (Just x) f = f x augment Nothing _ = Nothing combine Nothing y = y combine x _ = x instance Computation Failable where success = Success failure = Fail augment (Success x) f = f x augment (Fail s) _ = Fail s combine (Fail _) y = y combine x _ = x -} instance Monad Failable where return = Success fail = Fail (>>=) (Success x) f = f x (>>=) (Fail s) _ = Fail s mplus (Fail _) y = y mplus x _ = x {- instance Computation [] where success a = [a] failure = const [] augment l f = concat (map f l) combine = (++) searchAll g@(Graph vl el) src dst | src == dst = success [src] | otherwise = search' el where search' [] = failure "no path" search' ((u,v,_):es) | src == u = (searchAll g v dst `augment` (success . (u:))) `combine` search' es | otherwise = search' es -} searchAll g@(Graph vl el) src dst | src == dst = return [src] | otherwise = search' el where search' [] = fail "no path" search' ((u,v,_):es) | src == u = (searchAll g v dst >>= (return . (u:))) `mplus` search' es | otherwise = search' es -----Inline Attachment Follows----- _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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Re: [Haskell-cafe] Missing a "Deriving"?
by michael rice 01 Jun '09

01 Jun '09

Got it. Thanks! Michael --- On Mon, 6/1/09, Daniel Fischer <daniel.is.fischer(a)web.de> wrote: From: Daniel Fischer <daniel.is.fischer(a)web.de> Subject: Re: [Haskell-cafe] Missing a "Deriving"? To: haskell-cafe(a)haskell.org Date: Monday, June 1, 2009, 1:51 PM Am Montag 01 Juni 2009 19:02:36 schrieb michael rice: > All good so far, but then tried to convert Failable from Computation to > Monad > > > instance Monad Failable where > return = Success > fail = Fail > >>= (Success x) f = f x > >>= (Fail s) _ = Fail s > mplus (Fail _) y = y > mplus x _ = x > > > and got the following error. > > > Prelude> :l graph5 > [1 of 1] Compiling Main ( graph5.hs, interpreted ) > > graph5.hs:34:4: parse error on input `>>=' > Failed, modules loaded: none. > Prelude> > When you use an operator in prefix position, you must enclose it in parentheses, like you must enclose a function in backticks if you use it infix. So the definition of (>>=) should read (>>=) (Success x) f = f x (>>=) (Fail s) _ = Fail s or, defining it in infix position, (Success x) >>= f = f x (Fail s) >>= _ = Fail s > > Complete code follows. > > Michael > _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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Error message reform (was: Strange type error with associated type synonyms)
by Max Rabkin 01 Jun '09

01 Jun '09

On Wed, May 27, 2009 at 11:05 PM, Bulat Ziganshin <bulat.ziganshin(a)gmail.com> wrote: > i mean just changing the words to make obvious what type was got in > what way. and check it on beginners who don't yet read your > explanations, for example teachers may test it on their students > > my English is limited... for example, it may be like this: > > read x > > Error: type of x is Integer > while type of read argument should be String For me, this word order is an improvement, but it doesn't help to leave out the words "expected" and "inferred". To me, those words add explanation. Of course they don't help if you don't know what they mean. I think they add something because they explains where the types "came from". When I get type errors, I often think "why do you say True has type Bool" (of course in most cases it is not so transparent). Although the compiler doesn't say *why* it inferred that type, at least it says it did infer it. I prefer this wording: The inferred type of `True' is `Bool', while the type of the first argument of `f' should be `Int'. In the expression: f True I prefer all three to Hugs's ERROR - Type error in application *** Expression : f True *** Term : True *** Type : Bool *** Does not match : Int --Max

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Re: [Haskell-cafe] Missing a "Deriving"?
by michael rice 01 Jun '09

01 Jun '09

I went back and tried to convert the YAHT example to Monad, importing Monad, commenting out all but the data descriptions and the searchAll function, and finally replacing success, failure, augment, and combine in the searchAll function with return, fail, >>=, and mplus. *Main> let g = Graph [(1,'a'),(2,'b'),(3,'c'),(4,'d')] [(1,2,'p'),(2,3,'q'),(1,4,'r'),(4,3,'s')] *Main> searchAll g 1 3 :: [[Int]] [[1,2,3],[1,4,3]] *Main> searchAll g 1 3 :: Maybe [Int] Just [1,2,3] *Main> searchAll g 3 1 :: Maybe [Int] Nothing *Main> searchAll g 3 1 :: [[Int]] [] All good so far, but then tried to convert Failable from Computation to Monad instance Monad Failable where return = Success fail = Fail >>= (Success x) f = f x >>= (Fail s) _ = Fail s mplus (Fail _) y = y mplus x _ = x and got the following error. Prelude> :l graph5 [1 of 1] Compiling Main ( graph5.hs, interpreted ) graph5.hs:34:4: parse error on input `>>=' Failed, modules loaded: none. Prelude> Complete code follows. Michael ========================= import Monad data Failable a = Success a | Fail String deriving (Show) data Graph v e = Graph [(Int,v)] [(Int,Int,e)] {- class Computation c where success :: a -> c a failure :: String -> c a augment :: c a -> (a -> c b) -> c b combine :: c a -> c a -> c a instance Computation Maybe where success = Just failure = const Nothing augment (Just x) f = f x augment Nothing _ = Nothing combine Nothing y = y combine x _ = x instance Computation Failable where success = Success failure = Fail augment (Success x) f = f x augment (Fail s) _ = Fail s combine (Fail _) y = y combine x _ = x -} instance Monad Failable where return = Success fail = Fail >>= (Success x) f = f x >>= (Fail s) _ = Fail s mplus (Fail _) y = y mplus x _ = x {- instance Computation [] where success a = [a] failure = const [] augment l f = concat (map f l) combine = (++) searchAll g@(Graph vl el) src dst | src == dst = success [src] | otherwise = search' el where search' [] = failure "no path" search' ((u,v,_):es) | src == u = (searchAll g v dst `augment` (success . (u:))) `combine` search' es | otherwise = search' es -} searchAll g@(Graph vl el) src dst | src == dst = return [src] | otherwise = search' el where search' [] = fail "no path" search' ((u,v,_):es) | src == u = (searchAll g v dst >>= (return . (u:))) `mplus` search' es | otherwise = search' es

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MySQL, CouchDB, and Haskell
by Henry Laxen 01 Jun '09

01 Jun '09

Dear Group, I've spent the last few days trying to convert a bunch of mysql tables into couchdb using haskell, and I've documented my efforts, in case anyone else intends to wander in similar waters. The tutorial is at: http://maztravel.com/haskell/mySqlToCouchDB.html comments welcome here at the list or to me privately at nadine.and.henry -AT- pobox.com Best wishes, Henry Laxen

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How to implement this? A case for scoped record labels?
by ntupel＠googlemail.com 01 Jun '09

01 Jun '09

Hi, I would like to get some advice on how to best implement a protocol. The protocol in question is "Bayeux": http://svn.cometd.org/trunk/bayeux/bayeux.html. The details don't matter here - it defines a couple of requests and responses in JSON format, basically JSON objects with different properties, some of which are shared by all (e.g. "channel") and some which are specific for certain kinds of requests/responses (e.g. "subscription"). To give an example, a connect request would look like this: [ { "channel": "/meta/connect", "clientId": "Un1q31d3nt1f13r", "connectionType": "long-polling" } ] Now I leave the actual JSON parsing to the excellent Text.JSON library. My problem is how to get the types right. At first I started with a big discriminated union, e.g. data BayeuxMessage = HandshakeRequest { channel :: String , ... } | HandshakeResponse { channel :: String, successful :: Bool, ... } | ... This way I could create BayeuxMessage values by copying the Text.JSON parsed values over. However what I don't like is that many selector functions, e.g. successful, are only partial and using them with a BayeuxMessage value constructed with HandshakeRequest for example will result in a runtime error. So I think it would be better to have individual types for the protocol requests/responses, e.g. data HandshakeRequest = HandshakeRequest { channel :: String , ... } data HandshakeResponse = HandshakeResponse { channel :: String, successful :: Bool, ... } ... data BayeuxMessage = HSReq HandshakeRequest | HSRes HandshakeResponse ... This however does not work because record selectors have module scope, so the compiler will complain that channel et. al. are defined multiple times. As a workaround I could put each type into its own module, but at least GHC requires a file per module (which is *very* inconvenient IMO). If we would have scoped labels (e.g. like proposed here: http://legacy.cs.uu.nl/daan/pubs.html#scopedlabels) it seems like it would have been straightforward. So certainly I am missing something and there is a better way to design this. Hence this e-mail. I welcome any advice how this would best be done in Haskell with GHC. Many thanks, nt

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GUI and background processing
by Dmitry V'yal 01 Jun '09

01 Jun '09

Greetings, fellow haskellers. Currently I'm writing some kind of web crawler in haskell with gtk2hs gui. All network operations are run in separate thread, but sometimes input from user is needed. Afaik, gtk2hs is not thread safe, so I came up with following: I create two mvars, A and B, one for question, one for answer. Worker thread puts question in A and blocks trying to take B. Gui thread checks for question by polling with tryTakeMVar. Then question is ready, it asks user and puts his answer into B. Worker awakens and continues processing. Also, for progress report, I use a Chan which is written to by worker and is read by polling by gui thread. This scheme works, but I don't like it. All these mvars and chans create too much clutter, and polling looks rather ugly too. Is there a better way for implementing similar patterns of interaction? Best regards, Dmitry.

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RE: iota
by Paul Keir 01 Jun '09

01 Jun '09

That is quite spectacular. I revised my knowledge of sequence with a little function, akin to "sequence [xs1,xs2]": seq2 xs1 xs2 = do x1 <- xs1 x2 <- xs2 return [x1,x2] > seq2 [0,1] [0,1,2] > [[0,0],[0,1],[0,2],[1,0],[1,1],[1,2]] I like your point-free style too; and that's a nice use of pred. Many thanks, Paul The iota function you're looking for can be a whole lot simpler if you know about monads (list monad in particular) and sequence. For lists, sequence has the following behaviour: sequence [xs1,xs2, ... xsn] = [[x1,x2, ... , xn] | x1 <- xs1, x2 <- xs2, ... , xn <- xsn] Using this, you can reduce your iota function to a powerful one-liner: iota = sequence . map (enumFromTo 0 . pred) Kind regards, Raynor Vliegendhart From: Paul Keir Sent: 01 June 2009 10:01 To: haskell-cafe(a)haskell.org Subject: iota Hi all, I was looking for an APL-style "iota" function for array indices. I noticed "range" from Data.Ix which, with a zero for the lower bound (here (0,0)), gives the values I need: > let (a,b) = (2,3) > index ((0,0),(a-1,b-1)) > [(0,0),(0,1),(0,2),(1,0),(1,1),(1,2)] However, I need the results as a list of lists rather than a list of tuples; and my input is a list of integral values. I ended up writing the following function instead. The function isn't long, but longer than I first expected. Did I miss a simpler approach? iota :: (Integral a) => [a] -> [[a]] iota is = let count = product is tups = zip (tail $ scanr (*) 1 is) is buildRepList (r,i) = genericTake count $ cycle $ [0..i-1] >>= genericReplicate r lists = map buildRepList tups in transpose lists > length $ iota [2,3,4] > 24 Thanks, Paul

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