November 2015 - Haskell-Cafe

Who's hiring, HS cafe edition Re: Haskell Engineer Needed for Machine Learning Group
by Carter Schonwald 03 Nov '15

03 Nov '15

My team at "megacorp " is hiring. Our main project right now is a domain specific sibling to Agda / idris that runs on top of strongly consistent replicated db that we're also in the process of iterating on. A colleague and I will be at Hac Phi this weekend, and we're also helping sponsor some of the catering costs. Talk to us if you're there! On Tuesday, November 3, 2015, Kim-Ee Yeoh <ky3(a)atamo.com> wrote: > > On Tue, Nov 3, 2015 at 6:24 PM, Adam Bergmark <adam(a)bergmark.nl > <javascript:_e(%7B%7D,'cvml','adam(a)bergmark.nl');>> wrote: > >> There are plenty of other places where people who don't want to work with >> haskell can look for employment. > > > You've lost me. Haskell-cafe is where people gather who want to work with > haskell. > > -- Kim-Ee >

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Re: [Haskell-cafe] process package with compilers besides GHC
by Malcolm Wallace 03 Nov '15

03 Nov '15

Clean-up is generally good. I have not maintained any Haskell compiler for about 5 years now, so I think it is safe to say you can go ahead. nhc98 did indeed build the 'process' package as part of its library set, but most recently at version 1.0.1.4. Regards, Malcolm On 2 Nov 2015, at 16:21, Michael Snoyman wrote: > Forwarding directly to you, in case you're not reading the mailing lists. > > ---------- Forwarded message ---------- > From: Michael Snoyman <michael(a)snoyman.com> > Date: Mon, Nov 2, 2015 at 8:10 AM > Subject: process package with compilers besides GHC > To: Haskell Cafe <haskell-cafe(a)haskell.org>, libraries <libraries(a)haskell.org> > > > I'm currently in the process* of a cleanup of the process package to reduce the usage of CPP conditionals in the code. Currently, as Simon PJ pointed out, it's quite difficult to follow the logic between Windows and POSIX platforms. During the cleanup, I've noticed that: > > 1. There's also quite a bit of complication around the __GLASGOW_HASKELL__ conditionals, intended to make the package compilable with other compilers > 2. There seems to be no way that - in its current state - the package could be compilable with a compiler besides GHC > > I wanted to reach out and see if anyone is currently using the process package on a compiler besides GHC, and if so, get information on whether recent releases also work, and get some kind of automated testing in place to prevent regressions. If there are no such users, I'm likely to remove the (presumably non-working) conditionals. > > * No pun intend >

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remove account
by Richard Torruellas III 03 Nov '15

03 Nov '15

Can you remove the account associated with this email address. I did not realize passwords were stored in plain text before sending (I missed the small print on the page) Thanks

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'Associative' order of calling
by Charles Durham 03 Nov '15

03 Nov '15

Is there a name for a fold that promises to call a function such that only an associative function will always return the same result. Or in other words, it has the property that it promises to call a function "associatively" on a set of data? Thanks! Charlie

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process package with compilers besides GHC
by Michael Snoyman 03 Nov '15

03 Nov '15

I'm currently in the process* of a cleanup of the process package to reduce the usage of CPP conditionals in the code. Currently, as Simon PJ pointed out, it's quite difficult to follow the logic between Windows and POSIX platforms. During the cleanup, I've noticed that: 1. There's also quite a bit of complication around the __GLASGOW_HASKELL__ conditionals, intended to make the package compilable with other compilers 2. There seems to be no way that - in its current state - the package could be compilable with a compiler besides GHC I wanted to reach out and see if anyone is currently using the process package on a compiler besides GHC, and if so, get information on whether recent releases also work, and get some kind of automated testing in place to prevent regressions. If there are no such users, I'm likely to remove the (presumably non-working) conditionals. * No pun intend

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ANN: dimensional-1.0 for statically checked physical dimensions
by Douglas McClean 02 Nov '15

02 Nov '15

Hello Haskellers, Björn Buckwalter and I are pleased to announce the release to Hackage of version 1.0 of the dimensional library, which statically tracks physical dimensions in Haskell code, as in the example below, preventing dimensional mistakes and requiring explicit documentation of units where raw values are exchanged with external systems. {-# LANGUAGE NoImplicitPrelude #-} module Example where import Numeric.Units.Dimensional.Prelude import Numeric.Units.Dimensional.NonSI -- a function that computes with dimensional values escapeVelocity :: (Floating a) => Mass a -> Length a -> Velocity a escapeVelocity m r = sqrt (two * g * m / r) where two = 2 *~ one g = 6.6720e-11 *~ (newton * meter ^ pos2 / kilo gram ^ pos2) >>> let re = 6372.792 *~ kilo meter >>> let me = 5.9742e24 *~ kilo gram >>> let ve = escapeVelocity me re >>> ve -- Show defaults to SI base units 11184.537332296259 m s^-1 >>> showIn (mile / hour) ve -- but we can show in other units "25019.09746845083 mi / h" >>> let vekph = ve /~ (kilo meter / hour) -- and extract raw values when needed 40264.33439626653 This version is a major upgrade, consolidating features from the classic dimensional package and the dimensional-tf package. It takes advantage of the DataKinds and ClosedTypeFamilies extensions in GHC 7.8 to offer even safer types with a nearly identical interface. Also included: - Units carry names which can be combined by multiplication, division, and (only where appropriate) application of metric prefixes. You can use expressions like: showIn (milli meter / second) timeTravelSpeed to get "39339.52 mm / s" - Exact conversion factors between units are available, even when those conversion factors involve multiples of pi, thanks to the exact-pi library - The dimensionally-polymorphic siUnit term represents the coherent SI base unit of any dimension, which can be convenient for wrapping and unwrapping quantities in some contexts. - Storable and Unbox instances for Quantity types are available thanks to the efforts of Alberto Valverde González. - The Numeric.Units.Dimensional.Dynamic module offers types for safely manipulating quantities and units whose dimensions are not known statically. Also available is a term-level representation for dimensions. - Several other missing instances have been provided, including Bounded, Data, and Typeable instances. - New commonly used US customary units have been added, including US fluid measures and the knot. We have several other development efforts underway, including a type-checker plugin inspired by Adam Gundry's work, and on which he has provided valuable advice, which we hope will lead to a clean library for dimensionally typed linear algebra. Comments and contributions are welcome at http://github.com/bjornbm/dimensional-dk. (The repository name is a carryover from the name we were using while developing this version.) One particularly welcome contribution would be assistance with developing a patch for GHC issue 10391 <https://ghc.haskell.org/trac/ghc/ticket/10391>. Cheers, Doug McClean

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Why does disambiguation in RebindableSyntax sometimes requires type signatures?
by Jan Bracker 02 Nov '15

02 Nov '15

Hello, I am currently playing around with RebindableSyntax and having several bind/return/sequence functions in scope at the same time. I thought that it would be enough to just pick the right one to use in each do-block by using a "where" or a "let". Surprisingly, I get some type related issues I can only fix by adding in some type signatures, but I don't understand why these signatures are actually necessary. Here is my example program: {-# LANGUAGE RebindableSyntax #-} {-# LANGUAGE DataKinds #-} {-# LANGUAGE TypeOperators #-} import Prelude import qualified Prelude as P import qualified Control.Effect as E import Control.Effect.State ifThenElse :: Bool -> a -> a -> a ifThenElse True t e = t ifThenElse False t e = e main :: IO () main = do return () where return = P.return data Tree = Leaf Int | Branch Tree Tree process :: Tree -> State '[ "flatten" :-> Bool :! 'R ] (Either Tree [Int]) process (Leaf i) = do flatten <- get (Var :: (Var "flatten")) if flatten then return $ Right [i] else return $ Left $ Leaf i where --(>>=) :: (E.Inv State f g) => State f a -> (a -> State g b) -> State (E.Plus State f g) b (>>=) = (E.>>=) (>>) :: (E.Inv State f g) => State f a -> State g b -> State (E.Plus State f g) b (>>) = (E.>>) return = E.return fail = E.fail process (Branch tl tr) = do eitherL <- process tl eitherR <- process tr case (eitherL, eitherR) of (Left l, Left r) -> return $ Left $ Branch l r (Right l, Right r) -> return $ Right $ l ++ r where (>>=) :: (E.Inv State f g) => State f a -> (a -> State g b) -> State (E.Plus State f g) b (>>=) = (E.>>=) (>>) :: (E.Inv State f g) => State f a -> State g b -> State (E.Plus State f g) b (>>) = (E.>>) return = E.return fail = E.fail The program uses the "effect-monad" package in version 0.6.1. 1) The type signatures in the "where" following each do-block of the "process" function are required. If I remove the type signature of the sequence functions I get a type error of the following nature: examples/effect/Test.hs:33:16: Could not deduce (E.Inv m0 f0 g0) arising from a use of ‘E.>>’ Relevant bindings include (>>) :: m0 f0 a -> m0 g0 b -> m0 (E.Plus m0 f0 g0) b (bound at examples/effect/Test.hs:33:9) In the expression: (E.>>) In an equation for ‘>>’: (>>) = (E.>>) In an equation for ‘process’: process (Leaf i) = do { flatten <- get (Var :: Var "flatten"); if flatten then return $ Right [...] else return $ Left $ Leaf i } where (>>=) = (E.>>=) (>>) = (E.>>) return = E.return fail = E.fail examples/effect/Test.hs:33:16: No instance for (E.Effect m0) arising from a use of ‘E.>>’ In the expression: (E.>>) In an equation for ‘>>’: (>>) = (E.>>) In an equation for ‘process’: process (Leaf i) = do { flatten <- get (Var :: Var "flatten"); if flatten then return $ Right [...] else return $ Left $ Leaf i } where (>>=) = (E.>>=) (>>) = (E.>>) return = E.return fail = E.fail Which I interpret as the inability to infer the "E.Effect" and "E.Inv" constraints that are implied by the use of "E.>>". But why can't those constraints be inferred correctly? Shouldn't a definition like "(>>) = (E.>>)" just propagate the type signature and specialize it as needed? 2) If I remove the type signature for the bind operation, I get the following type error: examples/effect/Test.hs:38:3: Couldn't match type ‘'["flatten" :-> (Bool :! 'R)]’ with ‘'[]’ Expected type: State '["flatten" :-> (Bool :! 'R)] (Either Tree [Int]) -> (Either Tree [Int] -> State '[] (Either Tree [Int])) -> State '[] (Either Tree [Int]) Actual type: State '["flatten" :-> (Bool :! 'R)] (Either Tree [Int]) -> (Either Tree [Int] -> State '[] (Either Tree [Int])) -> State (E.Plus State '["flatten" :-> (Bool :! 'R)] '[]) (Either Tree [Int]) In a stmt of a 'do' block: eitherR <- process tr In the expression: do { eitherL <- process tl; eitherR <- process tr; case (eitherL, eitherR) of { (Left l, Left r) -> return $ Left $ Branch l r (Right l, Right r) -> return $ Right $ l ++ r } } In an equation for ‘process’: process (Branch tl tr) = do { eitherL <- process tl; eitherR <- process tr; case (eitherL, eitherR) of { (Left l, Left r) -> return $ Left $ Branch l r (Right l, Right r) -> return $ Right $ l ++ r } } where (>>=) = (E.>>=) (>>) :: (E.Inv State f g) => State f a -> State g b -> State (E.Plus State f g) b (>>) = (E.>>) return = E.return Which tells me that GHC is expecting the wrong type, but inferring the correct type. Again I don't see why this wrong type is expected and the right type is ignored. In either case, why does adding or removing the type signature for the local definitions make a difference at all? I suspect the issue has to do with the language extensions I enabled or the type-level computations that are done by the "effect-monad" package, but I cannot find a satisfying answer. Does anybody have a good explanation? I am working with GHC 7.10.2. Best, Jan

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simple library for HTML form generation and processing
by Henning Thielemann 02 Nov '15

02 Nov '15

I posted the following already to web-devel(a)haskell.org but it seems to be relatively quiet there. ----- I like to get your advice about a simple library for web form generation and processing. So far I tried digestive-functors and reform. Both of them are advertised as being good in separating data from view. However, I am interested in one-off forms, that is, form data will always be rendered in one way. Thus I would like to save the efforts to combine data and view after they have been carefully separated. Formerly I have also used autolib-cgi: http://autolat.imn.htwk-leipzig.de/haddock/autolib-cgi-1.2/ It allowed me to generate the form and fetch and validate the data in one (monadic) go. The blaze-markup type MarkupM is a monad anyway, thus it seems to be straight-forward to use the monadic results for the data entered to the form. Is there a library that supports this style of web form handling?

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testing for exceptions
by Roelof Wobben 02 Nov '15

02 Nov '15

Hello, I have made this exercise which can be found at the Craft of Functional Programming book. -- exercise 32 -- Suppose we have to raise 2 to the power n. If n is even, 2*m say, then -- 2n = 22*m = (2m)2 -- If n is odd, 2*m+l say, then -- 2n = 22*m+l = (2n)2*2 -- Give a recursive function to compute 2n which uses these insights. f2 :: Integer -> Integer f2 n | n < 0 = error "This will only run for positive numbers" | n == 0 = 1 | even n = f2 ( n `div` 2) ^ 2 | odd n = (f2 ( n `div` 2) ^ 2) * 2 Now I have to make Hunit tests for it, But is there a way I can test if the error message is shown when a negative number is being used ? Roelof

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what do I have to do exactlry with this exercises
by Roelof Wobben 02 Nov '15

02 Nov '15

Hello, Im self studing Haskell with the Craft o ffunctional programmimg of Hutton. Now I see two exercises that I do not understand what is really the purpose here. The two exercises are : 4.21 Given a function f of type Integer -> Integer give a recursive definition of a function of type Integer -> Integer which on input n returns the maximum of the values f 0, f 1, ..., f n. You might find the max function defined in Section 3.4 useful. To test this function, add to your script a definition of some values of f thus: f 0 = 0 f 1 = 44 f 2 = 17 f _ = 0 and so on; then test your function at various values. 4.22 Given a function f of type Integer -> Integer give a recursive definition of a function of type Integer -> Bool which on input n returns True if one or more of the values f 0, f 1, ..., f n is zero and False otherwise.

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