December 2005 - Haskell

Tools for web-based Haskell editing & compilation?
by Conal Elliott 25 Dec '05

25 Dec '05

Has anyone set up GHC to run on a server with modules created, saved, edited, imported, and executed via a web interface? Are there tools existing or under development that support some or all of this scenario? I'm working on a new implementation, "Pajama", of the Pan functional image synthesis DSEL, this time with the compiler targetting Java applets. (The image compiler is implemented in Haskell and generates Java code.) Besides running image effects, I'd also like to allow people to author new effects without having any software installed (GHC, the Pajama compiler, or the Java compiler). For starters, I'm thinking of an editor applet (running in a web page) and a compile server. Ideally, the editor would do syntax coloring and checking, show type annotations, etc, though it doesn't have to do so initially. When a module compiles successfully, the user would select definitions that would get run to generate Java code, which would then be compiled on the server and downloaded to the client via an html applet tag. Thanks, - Conal

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RE: [Haskell] Hlist distribution -- compiler error in TIP
by Ralf Lammel 24 Dec '05

24 Dec '05

Dear Frank, HList is definitely tested with 6.4. See transcript below. My guess would be that you are perhaps not running with options as stated in the Makefile. Anyway, if your problems persist, I am happy to take this offline. Merry Christmas, Ralf ralfla@ralfla02 ~/projects/HList/src $ ghc --version The Glorious Glasgow Haskell Compilation System, version 6.4 ralfla@ralfla02 ~/projects/HList/src $ make test ghci -fglasgow-exts -fallow-overlapping-instances -fallow-undecidable-instances MainGhcGeneric1.hs -v0 < Main.in > MainGhcGeneric1.out diff -b MainGhcGeneric1.out MainGhcGeneric1.ref ghci -fglasgow-exts -fallow-overlapping-instances -fallow-undecidable-instances MainGhcTTypeable.hs -v0 < Main.in > MainGhcTTypeable.out diff -b MainGhcTTypeable.out MainGhcTTypeable.ref runhugs -98 +o MainHugsTTypeable.hs < Main.in > MainHugsTTypeable.out diff -b MainHugsTTypeable.out MainHugsTTypeable.ref ghci -fglasgow-exts -fallow-overlapping-instances -fallow-undecidable-instances MainPosting-040607.hs -v0 < Main.in > MainPosting-040607.out diff -b MainPosting-040607.out MainPosting-040607.ref ghci -fglasgow-exts -fallow-overlapping-instances -fallow-undecidable-instances MainGhcGeneric2.hs -v0 < Main.in > MainGhcGeneric2.out diff -b MainGhcGeneric2.out MainGhcGeneric2.ref ghci -fglasgow-exts -fallow-overlapping-instances -fallow-undecidable-instances MainGhcGeneric3.hs -v0 < Main.in > MainGhcGeneric3.out diff -b MainGhcGeneric3.out MainGhcGeneric3.ref ralfla@ralfla02 ~/projects/HList/src > -----Original Message----- > From: haskell-bounces(a)haskell.org [mailto:haskell-bounces@haskell.org] On > Behalf Of Frank > Sent: Saturday, December 24, 2005 8:46 AM > To: haskell(a)haskell.org > Subject: [Haskell] Hlist distribution -- compiler error in TIP > > I just got the Hlist distribution but could not compile the example files > With ghc 6.4 (I use ghci - in eclipse - I did not use ghc with the > makefile). > > I got the following compile time error on module TIP: > > > C:\gisCodev13\HeterogenousList\src/TIP.hs:247:22: > Could not deduce (HType2HNat e l n1, > HDeleteAtHNat n1 l l'1, > HOccurs e (TIP l), > HOccurs e2 (TIP l'1)) > from the context (HOccurs e1 (TIP l), > HType2HNat e1 l n, > HDeleteAtHNat n l l', > HOccurs e2 (TIP l'), > HOccurs e2 (TIP l), > HType2HNat e2 l n', > HDeleteAtHNat n' l l'', > HOccurs e1 (TIP l'')) > arising from use of `y' at > C:\gisCodev13\HeterogenousList\src/TIP.hs:247:22 > Probable fix: > add (HType2HNat e l n1, > HDeleteAtHNat n1 l l'1, > HOccurs e (TIP l), > HOccurs e2 (TIP l'1)) > to the type signature(s) for `tuple' > or add an instance declaration for (HOccurs e (TIP l), HOccurs e2 > (TIP > l'1)) > In the definition of `tuple': > tuple (TIP l) > = let > x = hOccurs (TIP l) > l' = hDeleteAtProxy (toProxy x) l > y = hOccurs (TIP l') > in (x, y) > > > Can anybody quickly see, what is wrong? > > > > Andrew U. Frank > Professor, Head of Department > Geoinformation and Cartography E127 phone: +43 1 588 01 12710 > TU Vienna secr. +43 1 588 01 12700 > Gusshausstrasse 27-29 fax +43 1 588 01 12799 > A-1040 Vienna Austria cellular phone +43 676 41925 72 > http://www.geoinfo.tuwien.ac.at/persons/frank/frank.html > skype:AndrewUFrank > > _______________________________________________ > Haskell mailing list > Haskell(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell

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Hlist distribution -- compiler error in TIP
by Frank 24 Dec '05

24 Dec '05

I just got the Hlist distribution but could not compile the example files With ghc 6.4 (I use ghci - in eclipse - I did not use ghc with the makefile). I got the following compile time error on module TIP: C:\gisCodev13\HeterogenousList\src/TIP.hs:247:22: Could not deduce (HType2HNat e l n1, HDeleteAtHNat n1 l l'1, HOccurs e (TIP l), HOccurs e2 (TIP l'1)) from the context (HOccurs e1 (TIP l), HType2HNat e1 l n, HDeleteAtHNat n l l', HOccurs e2 (TIP l'), HOccurs e2 (TIP l), HType2HNat e2 l n', HDeleteAtHNat n' l l'', HOccurs e1 (TIP l'')) arising from use of `y' at C:\gisCodev13\HeterogenousList\src/TIP.hs:247:22 Probable fix: add (HType2HNat e l n1, HDeleteAtHNat n1 l l'1, HOccurs e (TIP l), HOccurs e2 (TIP l'1)) to the type signature(s) for `tuple' or add an instance declaration for (HOccurs e (TIP l), HOccurs e2 (TIP l'1)) In the definition of `tuple': tuple (TIP l) = let x = hOccurs (TIP l) l' = hDeleteAtProxy (toProxy x) l y = hOccurs (TIP l') in (x, y) Can anybody quickly see, what is wrong? Andrew U. Frank Professor, Head of Department Geoinformation and Cartography E127 phone: +43 1 588 01 12710 TU Vienna secr. +43 1 588 01 12700 Gusshausstrasse 27-29 fax +43 1 588 01 12799 A-1040 Vienna Austria cellular phone +43 676 41925 72 http://www.geoinfo.tuwien.ac.at/persons/frank/frank.html skype:AndrewUFrank

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ANN: hdbc-postgresql 0.6.0
by John Goerzen 24 Dec '05

24 Dec '05

I am pleased to announce the second database backend driver for HSQL, hdbc-postgresql. This driver obviously supports PostgreSQL databases ;-) Get it with: darcs get --partial --tag=hdbc-postgresql-0.6.0 \ http://darcs.complete.org/hdbc-postgresql -- John Goerzen Author, Foundations of Python Network Programming http://www.amazon.com/exec/obidos/tg/detail/-/1590593715

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Problem with constructing instances for a class Pair
by Frank 23 Dec '05

23 Dec '05

i want the operations on pairs to work for lists applying to each pair in the list. i prefer to achieve this with overloading, using classes and instances i cannot get from the code for the ordinary case: class P a where outl :: a b c -> b ... data P1 p q = P1 p q deriving (Show, Eq, Ord) -- a pair like constructor instance P P1 where outl (P1 p q) = p to code for the mapped case: instance P [P1] where .... gives -- - Kind error: `P1' is not applied to enough type arguments -- - In the instance declaration for `P [P1]' no other combination of parameters i can think of works for an instance of class P for lists. what do i not see? any help appreciated! andrew frank the remainder is the code i have tried: -} module PairTest where class P a where outl :: a b c -> b outr :: a b c -> c make :: b -> c -> a b c data P1 p q = P1 p q deriving (Show, Eq, Ord) type P2 = (,) instance P P2 where outl = fst outr = snd make = (,) instance P P1 where outl (P1 p q) = p outr (P1 p q) = q make = P1 p1list :: [P1 Int Bool] p1list = [P1 3 True, P1 4 False] qlist = fmap outr p1list -- or map --------------can this be done as overloading of outl, outr for lists? outlPl = fmap outl plist = outlPl p1list --instance P [P1] where -- outl = fmap outl -- - Kind error: `P1' is not applied to enough type arguments -- - In the instance declaration for `P [P1]' --instance P [P1 p q] where -- outl = fmap outl -- Kind error: Expecting kind `* -> * -> *', but `[P1 p q]' has kind `*' -- In the instance declaration for `P [P1 p q]' class P3 a b c where outl3 :: a b c -> b --instance P3 [P1 p q] -- - Kind error: Expecting kind `* -> * -> *', but `[P1 p q]' has kind `*' -- - In the instance declaration for `P3 [P1 p q]' Andrew U. Frank Professor, Head of Department Geoinformation and Cartography E127 phone: +43 1 588 01 12710 TU Vienna secr. +43 1 588 01 12700 Gusshausstrasse 27-29 fax +43 1 588 01 12799 A-1040 Vienna Austria cellular phone +43 676 41925 72 http://www.geoinfo.tuwien.ac.at/persons/frank/frank.html skype:AndrewUFrank

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Drawing charts via Haskell
by Gerbrand van Dieijen 23 Dec '05

23 Dec '05

Hello, I plan to develop a library in Haskell to draw charts, such as pie charts, bar charts and line charts, to visualize data. Functionality will be similar to gnuplot, or chart libraries that already exist for other languages. The chart library is for (part of) my master-thesis project and it is still in its planning phase. There are several graphic-libraries for Haskell, but I couldn't find any specifically for drawing charts. To avoid that I am reinventing the wheel, does anyone know of such a library that is already developed or being developed? Regards, -- Gerbrand van Dieijen

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ANN: HDBC 0.6.0
by John Goerzen 22 Dec '05

22 Dec '05

Hello, [ sorry, I accidentally posted an early draft of this on -cafe ] Version 0.6.0 of HDBC and the Sqlite3 bindings are now available. New features since Tuesday's 0.5.0 announcement include: * New type system for marshalling different Haskell types back and forth * New support for querying meta-information about the server * Much improved documentation * Fix for a race condition in the Sqlite3 binding * New fetchAllRows function that returns a lazy list of result rows (think hGetContents for database queries) I will shortly begin work on a PostgreSQL binding. Also, I plan to write a MissingH interface module. The main idea of that is to provide a SQL backend for the MissingH AnyDBM interface (which uses *dbm databases for persistent hashes). Combined with Sqlite, this provides a very interesting and easy to use persistent hash interface. Where to get stuff: API docs, quick link: http://darcs.complete.org/hdbc/doc/index.html HDBC general layer: darcs get --partial --tag=hdbc-0.6.0 http://darcs.complete.org/hdbc Sqlite3 driver: darcs get --partial --tag=hdbc-sqlite3-0.6.0 \ http://darcs.complete.org/hdbc-sqlite3 -- John

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A question about fundeps <-> GADT interaction
by Tomasz Zielonka 22 Dec '05

22 Dec '05

Hello! (This message is a literate Haskell module) > {-# OPTIONS -fglasgow-exts -fno-warn-missing-methods #-} > module Term where The papers on GADTs have an example showing how you can transform, traverse and evaluate ASTs (or terms) with more type safety. I've used such an approach in one of my applications and it works remarkably well. However, I would like to be able to "turn off" that type-safety in some parts of code, for example to separate parsing from typing. I thought I found a way to do this, because I was able to create Typed (with all consistency checking) and Untyped (without consistency checking) terms. Unfortunately I seem to be unable to write any useful function on such terms - GHC complains that there are type errors. I think this is because of the known problem with current GADT implementation in GHC, namely - bad GADT / type-classes interaction. I would be very happy to hear that it will be solved in GHC 6.6. I use an additional type parameter for Term that determines whether the Term is used as Typed or Untyped. Typed terms will have types (Term Typed Int), (Term Typed Bool), ..., but Untyped terms only one type: (Term Untyped ()) I use a multiparameter type-class with fundeps to describe a relation between the Typed/Untyped tag used, the types actually used in data constructors and the resulting "phantom" type. > class F f a b | f a -> b where > -- these methods are just for experiments in GHCi > a2b :: f -> a -> b > b2a :: f -> b -> a > data Typed = Typed > instance F Typed a a where > data Untyped = Untyped > instance F Untyped a () where > data Term f a where > Lit :: (F f Int int) => > Int -> Term f int > Succ :: (F f Int int) => > Term f int -> Term f int > IsZero :: (F f Int int, F f Bool bool) => > Term f int -> Term f bool > If :: (F f Bool bool, F f a a') => > Term f bool -> Term f a' -> Term f a' -> Term f a' Some examples: a typeable term > ex1 :: Term Untyped () > ex1 = > If (IsZero (Succ (Lit 0))) > (Lit 1) > (Lit 2) the same as ex1, but Typed > ex1' :: Term Typed Int > ex1' = > If (IsZero (Succ (Lit 0))) > (Lit 1) > (Lit 2) a term that has type bug, but will be accepted as Untyped > ex2 :: Term Untyped () > ex2 = > If (IsZero (Succ (Lit 0))) > (Lit 1) > (IsZero (Lit 2)) Here is the function that doesn't type check (you can comment it out to get the rest of code to compile). > -- A simple cast from typed terms to untyped terms > untype :: Term Typed a -> Term Untyped () > untype (Lit x) = Lit x > untype (IsZero t) = IsZero (untype t) > untype (Succ t) = Succ (untype t) > untype (If c t e) = If (untype c) (untype t) (untype e) The error given by GHC (for the commented "untype" function) is: T.hs:52:8: Couldn't match the rigid variable `a' against `Int' `a' is bound by the type signature for `untype' Expected type: Int Inferred type: a When using functional dependencies to combine F Typed a a, arising from the instance declaration at T.hs:13:0 F Typed Int a, arising from the pattern for `Lit' at T.hs:52:8-12 at T.hs:52:8-12 In the definition of `untype': untype (Lit x) = Lit x Failed, modules loaded: none. So it seems it can't infer that the 'a' in (F Typed Int a) is Int. At the same time... *T> :t a2b a2b :: (F f a b) => f -> a -> b *T> :t a2b Untyped a2b Untyped :: a -> () *T> :t a2b Typed a2b Typed :: b -> b -- see? it knows a and b are equal! *T> :t b2a Typed b2a Typed :: b -> b If there is another way to do this right now (conveniently, Oleg! ;-), I would be more than happy to hear about it. If this worked, it would be a cool trick and a nice example for GADT use. Let me know if it was proposed before. Best regards Tomasz -- I am searching for a programmer who is good at least in some of [Haskell, ML, C++, Linux, FreeBSD, math] for work in Warsaw, Poland

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Design guidance sought for Ranged Sets
by Paul Johnson 21 Dec '05

21 Dec '05

When I started the Ranged Sets library "infinite" sets (i.e. sets based on infinite lists of ranges) looked easy. The set primitives of union and intersection are simple merge algorithms, so they would work fine on infinite lists of ranges. Set difference is implemented as a combination of intersection and negation, and negation is just a simple rearrangement of the range end points, so that works too. However there are cases where this reasoning doesn't work. Suppose I have two ranged sets based on Range (BoundaryBelow n) (BoundaryBelow (n+1)) Set X contains these ranges for all even values of n>0 Set Y contains these ranges for all odd values of n>0 The union of these sets should be the single range Range (BoundaryBelow 1) BoundaryAboveAll In practice however the naive merge algorithm will never terminate because it has to merge an infinite number of ranges into the single range above. A similar problem occurs with intersection. The intersection of these sets should be empty, but again the merge algorithm will iterate forever looking for the first range in the set. As a workaround in 0.0.2 I've put a counter in. The intersection routine adds a null empty range after 10 empty iterations, and the union routine puts a break between touching ranges after 10 full iterations. Hence set membership tests are guaranteed to terminate in the presence of infinite lists. However the same cannot be said of all the functions. Set equality in particular has to evaluate the entire set in order to return True (although it does terminate at the first counter-example). The question of whether a particular expression is guaranteed to terminate is not trivial if it contains even one infinite list. So my question is this: are infinite sets worth bothering about? Should I continue to put in fixes to ensure termination in at least some cases? Or should I just declare that infinite lists of ranges are unsupported? Thanks, Paul.

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[ANNOUNCE] Ranged Sets 0.0.2
by Paul Johnson 21 Dec '05

21 Dec '05

Version 0.0.2 of the Ranged Sets library is now available at http://sourceforge.net/projects/ranged-sets/ The new release features infix operators, improved ability to handle infinite lists, and lots of QuickCheck properties for added confidence. Paul.

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