- Haskell-Cafe - Haskell.org

Type-class conditional behavior
by Nicholas Tung 09 May '11

09 May '11

Dear all, I'd like to write a function "maybeShow :: a -> Maybe String", which runs "show" if its argument is of class Show. The context and motivation for this are as follows. I have a GADT type which encapsulates abstract-value computation (or constants or error codes), a snippet of which is below. data AV t where AVLeft :: AV a -> AV (Either a b) This is used to implement an arrow transformer, and due to Arrows mapping all Haskell functions, I cannot put some kind of qualification on the constructor, like "AVLeft :: Show a => ...". Of course any replies are welcome, but I do need something implemented and stable. If there are GHC-compatible hacks, even an "unsafeShow :: a -> String", that'd be great. I'd also prefer not to branch on all types which could possibly be maybeShow's argument. (Concretely, if I have "newtype AVFunctor a b c = AVF (a (AV b) (AV c))", then the Arrow class declaration forces all types, c.f. variable b, to be potential variables of type AV), class (Category a) => Arrow a where arr :: (b -> c) -> a b c Thanks very much, Nicholas — https://ntung.com — CS major @ UC Berkeley p.s. I posted this question on StackOverflow if you care to get brownie points there, http://goo.gl/PrmYW p.s. 2 -- if there is a general "dump var" function in ghci, which does more than ":info", I'd love to know :)

6 6

Using Haskell to describe computer hardware.
by Serguey Zefirov 09 May '11

09 May '11

http://thesz.mskhug.ru/svn/hhdl/ - main repository and http://thesz.mskhug.ru/svn/hhdl/examples/Simple.hs - three simple examples and http://thesz.mskhug.ru/svn/hhdl/MIPS-example/ - an attempt to describe MIPS-alike CPU using Haskell. Not yet done, it passes only simplest of tests (it fetches commands), but if you take a look at it you'll get an idea of what is going on. The goal is to create something along the lines of Bluespec, but as a DSeL in Haskell. Right now I concentrate on single-clock circuits, as they comprise most of hardware logic. I hacked together transformation of not-so-complex Haskell function definition into Haskell functions that operate on infinite streams. Of course, I tried my best to support calling other functions, conditional operators, pattern matching, etc, because that's what makes Haskell great and Bluespec has them too. Below is little explanation of three simple examples. First comes very simple function: $(transform [d| f1 :: (ToWires a, Num a) => (a,a) -> a f1 (a,b) = a + b |]) After $(transform ...) you will have two functions: f1 and f1_S, the second one operates on streams. ToWires class in the context ensure that input and output values can be converted into arrays of bits when we'll get that part ready. The type of f1_S is: *Simple> :t f1_S f1_S :: (ToWires a[a9GC], Num a[a9GC]) => (S a[a9GC], S a[a9GC]) -> (S a[a9GC], S [String]) or put simply: *Simple> :t f1_S f1_S :: (ToWires a, Num a) => (S a, S a) -> (S a, S [String]) It accepts two streams and return two streams - value and logs. We could add messages into logs: $(transform [d| f2 :: (ToWires a, Num a) => (a,a) -> a f2 (a,b) = s where s = a + b _display_a = a _display_b = b _display_s = s |]) f2 is just like f1, but when ran, it will produce logs about execution along the values. f2 used by runningSum, which compute a sum of values seen before current cycle: $(transform [d| runningSum :: (ToWires a, Num a) => a -> a runningSum x = currentSum where nextSum = f2 (x,currentSum) currentSum = register 0 nextSum _display_currentSum = currentSum _display_x = x _display_nextSum = nextSum |]) runningSum uses latches. It latches nextSum to pass the value to the next cycle using "register" function. "register" accepts default value and a value to latch (stream of values to delay) . When the time will come for synthesizing circuits, our register's will become actual latches with reset and clk wires. This is how we test our runningSum_S with simple data: runningSumLogs :: IO () runningSumLogs = do mapM putStrLn $ map unlines $ take 5 $ toList_S $ snd $ runningSum_S (fromList_S [1::Int ..]) return () The output will be like the following: currentSum: 0 x: 1 nextSum: 1 nextSum_0 = Simple.f2 (x_1, currentSum_2): a: 1 b: 0 s: 1 currentSum: 1 x: 2 nextSum: 3 nextSum_0 = Simple.f2 (x_1, currentSum_2): a: 2 b: 1 s: 3 currentSum: 3 x: 3 nextSum: 6 nextSum_0 = Simple.f2 (x_1, currentSum_2): a: 3 b: 3 s: 6 Logs are nested and after label "nextSum_0 = Simple.f2 (x_1, currentSum_2):" we see logs from f2_S function. The code is in QUITE EXPERIMENTAL stage, but feel free to play and comment.

1 0

A long day with HXT
by nadine.and.henry＠pobox.com 09 May '11

09 May '11

Dear Group, I have just spent many hours staring at code and losing some hair. My hope is to save you the same experience someday. Here was my goal: Take some XML, like <photo url="somewhere" align="left" alt=""/> and replace the align attribute with a class attribute, but only if the value of the align attribute was not null. This led to to followed (hugely condensed) attempts: ------------------------------------------------------------------------ > {-# LANGUAGE Arrows #-} > import Text.XML.HXT.Core > > -- <photo url="somewhere" align="left" alt=""/> > testString = "<photo url=\"somewhere\" align=\"left\" alt=\"\"/>" > toClass :: b -> QName > toClass = const (mkName "class") > > > try f = runLA (xread >>> processTopDown f) testString > > test1 = try $ > hasAttrValue "align" (not.null) >>> > processAttrl (changeAttrName toClass `when` hasName "align" ) > test2 = try $ processAttrl > (changeAttrName toClass `when` hasAttrValue "align" (not.null)) > test3 = try $ processAttrl > (changeAttrName toClass `when` hasName "align") > `when` hasAttrValue "align" (not.null) > > run = do > print test1 > print test2 > print test3 > result of run: [NTree (XTag "photo" [NTree (XAttr "url") [NTree (XText "somewhere") []],NTree (XAttr "class") [NTree (XText "left") []],NTree (XAttr "alt") []]) []] [NTree (XTag "photo" [NTree (XAttr "url") [NTree (XText "somewhere") []],NTree (XAttr "align") [NTree (XText "left") []],NTree (XAttr "alt") []]) []] [NTree (XTag "photo" [NTree (XAttr "url") [NTree (XText "somewhere") []],NTree (XAttr "class") [NTree (XText "left") []],NTree (XAttr "alt") []]) []] test1 and test3 performed as desired, needless to say I started out with something similar to test2 and was almost crying because the align attribute didn't change. Why? In test1, I run through the tree and select the elements which have a non-null align attribute. (hasAttrValue "align" (not.null)) Then for each of these elements I process their attributes. (processAttrl) where I change the attribute name to "class" (changeAttrName toClass) WHEN the attribute name is "align" (`when` hasName "align") Result: Joy In test2, I run through the tree and process every attribute list (processAttrl) I change the name of the attribute to "class" (changeAttrName toClass) WHEN the align attribute is not null (`when` hasAttrValue "align" (not.null))) Result: Nothing changes, - No Joy In test3, I run through the tree process every attribute list (processAttrl) WHEN the align attribute is not null (`when` hasAttrValue "align" (not.null)) and in that case I change the attribute name to "class" (changeAttrName toClass) WHEN the attribute is named "align" (`when` hasName "align") Result: Joy After staring at test2 for more time that I would like to acknowledge, I (think) I realized the following. In test2 I thought the (hasAttrValue "align" (not.null)) was referring to the tree that was coming into the test, but actually it is referring to the list of attributes that is generated by processAttrl. This list of attributes does not itself have any attributes, thus the (hasAttrValue "align" (not.null)) code never succeeds, and thus the (changeAttrName toClass) is never executed. Lessons learned: I'm not really sure, but it is these kind of subtleties that make me sometimes long for my old friend Perl. Best wishes, Henry Laxen -- Nadine & Henry Laxen Belle, Venus, Aphrodite 10580 N. McCarran Blvd. Adonis, Miss Parker & Jarod Suite 115-396 Via Alta # 6 Reno, Nevada Chapala, Jalisco, Mexico 89503-1896 CP 45900 The rest is silence. (Hamlet)

2 1

Re: [Haskell-cafe] [Haskell] ANN: syntactic-0.1
by Emil Axelsson 09 May '11

09 May '11

2011-05-07 09:52, Heinrich Apfelmus skrev: > Emil Axelsson wrote: >> I'm happy to announce the first release of syntactic: >> >> http://hackage.haskell.org/package/syntactic >> >> providing generic abstract syntax and utilities for embedded >> languages. >> >> To get an idea of what it's about, check out the tiny(!) >> implementation of (simplified) Feldspar in the Examples directory: >> >> http://projects.haskell.org/syntactic/Examples/MuFeldspar >> >> The library is intended to provide a growing set of utilities for >> embedded languages. In particular, I hope to be able to provide safe >> interfaces to various unsafe techniques for speeding things up. >> >> Comments, patches and clients are welcome! > > Looks very interesting! > > I'm currently working on a functional reactive programming DSL. Source code: > > http://tinyurl.com/reactive-banana-0-2-pushio > > The EventD type represents the abstract syntax tree (line 150), which > is then compiled in three phases (line 313) > > compile e ~= map compilePath =<< compileUnion =<< compileAccumB e > > If you look at the EventD type, you can see it contains a lot of > "internal constructors" (line 158) that are created in destroyed in > certain compilation phases. Ideally, I would make a new EventD type > for each compilation phase that doesn't have any superfluous > constructors. For instance, compileUnion removes the Union constructor. Hello Heinrich! Thanks for your interest! In principle, this is exactly the kind of thing you can do with syntactic -- if you can make your data type fit into the framework, that is. A compilation phase would then look something like this: phase :: AST (C1 :+: TheRest) -> AST (C2 :+: C3 :+: TheRest) This phase destroys the constructor C1 and introduces C2 and C3. Now, the question is whether your data type can be made to fit into the framework. The only problem I can see (but there may be more!) is that you'd have to treat EventD and BehaviorD as parts of the same data structure. I've pasted an example of how to do that here: http://hpaste.org/46525/ Here, the difference between Event and Behavior is denoted by the type constructors E and B respectively: type Event a = ASTF (Never :+: B2E :+: Pure :+: E2B) (E a) type Behavior a = ASTF (Never :+: B2E :+: Pure :+: E2B) (B a) You decide if this seems like an acceptable solution... > I'm also unhappy about some of the boilerplate. For instance, have a > look at the function goE in compileAccumB (line 210), it's just a > generic applicative traversal through the data type. Most likely, this boilerplate could be simplified using syntactic. > Would your library make me happier? Note that I have an additional > complication: I need to observe sharing and I need to support recursive AST. I plan to add observable sharing soon. Once that's is in place, it should -- I think/hope -- be easy to support cycles. This is not something we need in Feldspar, so it might take a while before I get around to supporting cycles, though. I'd be happy to give more assistance should you choose to try out syntactic for your library! Best regards / Emil

1 0

Apache license, any drawbacks for Haskell packages?
by Magnus Therning 09 May '11

09 May '11

Are there any drawbacks to using the Apache license for Haskell packages? /M -- Magnus Therning OpenPGP: 0xAB4DFBA4 email: magnus(a)therning.org jabber: magnus(a)therning.org twitter: magthe http://therning.org/magnus I invented the term Object-Oriented, and I can tell you I did not have C++ in mind. -- Alan Kay

4 4

repa Shape help
by briand＠aracnet.com 09 May '11

09 May '11

Howdy, as usual, the haskell type system complete defeats me in the simplest of applications: import Data.Array.Repa as A import Data.Array.Repa.Index import Data.Array.Repa.Shape as AS main = do let x = A.fromList (AS.shapeOfList [2, 2]) ([1.0, 2.0, 3.0, 4.0]::[Double]) putStrLn $ show x test_repa.hs:10:13: Ambiguous type variable `sh' in the constraint: `Shape sh' arising from a use of `show' at test_repa.hs:10:13-18 Probable fix: add a type signature that fixes these type variable(s) Failed, modules loaded: none. After much staring at the type signatures I finally figured out that adding a type annotation to x of : :: Array DIM2 Double would fix the problem, but I'm not completely clear as to why. after all fromList is typed: (Shape sh, Elt a) => sh -> [a] -> Array sh a Since it knows [a] is [Double] and sh must be - well I'm not really clear on what sh is supposed to be. therein lies my problem. Although it does seem that sh can be darn near anything, which is probably why it was ambiguous. At one point I had tried something like (2 :. 2) and got a whole host of errors for that too, except that DIM2 is defined in exactly that way, so it's not at all obvious why that didn't work. I was hoping someone could clarify on what's going on. Thanks, Brian

2 1

trying to cabal install lambdabot, failure on 'random' package
by Sean Perry 09 May '11

09 May '11

/usr/bin/ghc --make -package-name show-0.4.1.1 -hide-all-packages -fbuilding-cabal-package -i -idist/build -i. -idist/build/autogen -Idist/build/autogen -Idist/build -optP-include -optPdist/build/autogen/cabal_macros.h -odir dist/build -hidir dist/build -stubdir dist/build -package-id QuickCheck-2.4.0.1-5898cfb0116fc1d6ecef04b3f053c323 -package-id base-4.3.1.0-167743fc0dd86f7f2a24843a933b9dce -package-id random-1.0.0.3-749b78c54a8a99991b32dbb45d98a91b -package-id smallcheck-0.4-008eb21a2943dfdde26401dbc145c45b -package-id syb-0.3-8db7bc4339a1cb8a6f2a46c40447ef0b -O -Wall -XHaskell98 ShowQ ShowFun SimpleReflect <command line>: cannot satisfy -package-id random-1.0.0.3-749b78c54a8a99991b32dbb45d98a91b: random-1.0.0.3-749b78c54a8a99991b32dbb45d98a91b is shadowed by package random-1.0.0.3-57524486875e0c4c260dd22788921013 (use -v for more information) World file is already up to date. cabal: Error: some packages failed to install: lambdabot-4.2.3.2 depends on show-0.4.1.1 which failed to install. lambdabot-utils-4.2.1 failed during the building phase. The exception was: ExitFailure 1 readline-1.0.1.0 failed during the configure step. The exception was: ExitFailure 1 show-0.4.1.1 failed during the building phase. The exception was: ExitFailure 1 I have ghc 7.0.,3 and cabal $ cabal --version cabal-install version 0.10.2 using version 1.10.1.0 of the Cabal library From the recent Haskell Platform for OS X. Ideas?

4 6

Re: [Haskell-cafe] For Project Euler #24 you don't need to generate all the lexicographic permutations <Spoiler>
by KC 08 May '11

08 May '11

I see from the solutions on Project Euler others did think of this way but at least on my last IQ test I did get an 'A'; 95. I am a jenius. :D -- -- Regards, KC

1 0

For Project Euler #24 you don't need to generate all the lexicographic permutations <Spoiler>
by caseyh＠istar.ca 08 May '11

08 May '11

For Project Euler #24 you don't need to generate all the lexicographic permutations by Knuth's method or any other. You're only looking for the millionth lexicographic permutation of "0123456789" Problem 24 A permutation is an ordered arrangement of objects. For example, 3124 is one possible permutation of the digits 1, 2, 3 and 4. If all of the permutations are listed numerically or alphabetically, we call it lexicographic order. The lexicographic permutations of 0, 1 and 2 are: 012 021 102 120 201 210 What is the millionth lexicographic permutation of the digits 0, 1, 2, 3, 4, 5, 6, 7, 8 and 9? <Spoiler> <Spoiler> <Spoiler> Plan of attack: -- The "x"s are different numbers -- 0xxxxxxxxx represents 9! = 362880 permutations/numbers -- 1xxxxxxxxx represents 9! = 362880 permutations/numbers -- 2xxxxxxxxx represents 9! = 362880 permutations/numbers -- 20xxxxxxxx represents 8! = 40320 -- 21xxxxxxxx represents 8! = 40320 -- 23xxxxxxxx represents 8! = 40320 -- 24xxxxxxxx represents 8! = 40320 -- 25xxxxxxxx represents 8! = 40320 -- 26xxxxxxxx represents 8! = 40320 -- 27xxxxxxxx represents 8! = 40320 etc. <Spoiler> <Spoiler> -- lexOrder "0123456789" 1000000 "" lexOrder digits left s | len == 0 = s ++ digits | quot > 0 && rem == 0 = lexOrder (digits\\(show (digits!!(quot-1)))) rem (s ++ [(digits!!(quot-1))]) | quot == 0 && rem == 0 = lexOrder (digits\\(show (digits!!len))) rem (s ++ [(digits!!len)]) | rem == 0 = lexOrder (digits\\(show (digits!!(quot+1)))) rem (s ++ [(digits!!(quot+1))]) | otherwise = lexOrder (digits\\(show (digits!!(quot)))) rem (s ++ [(digits!!(quot))]) where len = (length digits) - 1 facLen = factorial len (quot,rem) = quotRem left facLen

2 1

no time profiling on my MacBookPro8,1
by Nicolas Frisby 08 May '11

08 May '11

For this vanilla program > module Main where > > main = print $ fib 40 > > fib 0 = 1 > fib 1 = 1 > fib n = fib (n - 1) + fib (n - 2) with these commands $ ghc -prof -auto-all -rtsopts -O --make Main.hs -o Main $ ./Main +RTS -p all of the %time cells in the generated Main.prof file are 0.0, as is the total time count (0.00 secs and 0 ticks). The %alloc cells seem normal. Andy Gill noticed that if you compile with -threaded, the %time cells seem normal. I scanned the GHC Trac tickets specific to Mac OS X, but saw no titles that looked similar. Is this a known issue? Thanks.

2 2