- Haskell-Cafe - Haskell.org

argument counts...
by Gregory Guthrie 21 Nov '17

21 Nov '17

I have a simple function: revFn (x:xs) = (revFn xs) . (x:) Of course GHCi correctly infers the type as: revFn :: [a] -> [a] -> c Adding the base case: revFn [] xs = xs Now gives an error; "Equations for 'revF' have different numbers of arguments" Of course this can be "fixed" by either adding the cancelled argument to the first clause, or converting the base case to only have one explicit argument, and a RHS of a lambda or identity function. But since the interpreter already correctly inferred that the first clause has two arguments (with only one explicit), why does it then ignore this and give an error when the second clause shows two explicit arguments? The types are all correct in either case - why require explicit arguments? Or, perhaps I am missing something simple?

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Re: [Haskell-cafe] Speculation, OT: Program a Spreadsheet
by Olaf Klinke 21 Nov '17

21 Nov '17

Hello Trent, let me suggest to step back and think about why people use spreadsheets. They provide access to data analysis functions, statistics and graphing that is otherwise missing (at least on) Windows systems. Serious number crunching is not done in spreadsheets, though. There is R, Matlab and the like. You should not try to comptete with that. The big selling point (in my eyes) of spreadsheets over scripting languages like R is that the data as well as the formulae and intermediate results are, well, spread out in front of your eyes. That is e.g. desirable when you need someone else to understand what you did and how you arrived at the answer given. But is it important that the intermediate result resides in cells C2 to C13? Certainly not. I wrote a small financial application in Haskell that would otherwise be done in spreadsheets. Haskell provided, via its type system, extra sanity checks and much more that spreadsheets don't provide. However, I often wished that there was a way to disassemble the computation into logical bits that can be formatted neatly and be presented to a human inspector. Haskell provides ways to do this, e.g. by mixing Haskell and LaTeX, or writing a small DSL with two interpreters, one for calculations, one for layout. So instead of enhancing a spreadsheet format I'd rather have a scripting language with a workbook file format à la Mathematica, where comments, code and results of computations are displayed together. Changing a formula triggers change in the results. (The workbooks I've seen don't do that.) Input data and intermediate results can be hidden or expanded via mouse click. Results can be traced back through the workbook's formulae up to the input data. (The latter is not always easy in spreadsheets.) One would need to research whether such a language exists. If not, I'd be happy to help develop it. Olaf

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Re: [Haskell-cafe] Speculation, OT: Program a Spreadsheet
by dominic＠steinitz.org 20 Nov '17

20 Nov '17

Here’s a presentation about a functional language embedded in Excel which I used when I worked at an investment bank https://www.youtube.com/watch?v=uICBMTeIl2U http://cufp.org/2009/fmd-functional-development-excel.html As far as I know the code is still in use (I recall there was something like 400 users in the bank) and was never open-sourced but it may provide some ideas given it was pretty successful internally. Dominic Steinitz dominic(a)steinitz.org http://idontgetoutmuch.wordpress.com

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implement a simple DSL meet problem with env data can't be changed
by mo jia 19 Nov '17

19 Nov '17

Base on the correct compiler desc on https://github.com/pa-ba/calc-comp Here is the source code module Lambda_define ( Name, Expr, eval )where import Prelude hiding (null, lookup, map, filter) import Data.HashMap.Lazy type Name = [Char] data Expr = Val Int | Add [Expr] | Var Name | Abs Name Expr | App Expr Expr | Define Name Expr | Begin [Expr] | Let (Name, Expr) Expr data Env = Root | Env { current :: HashMap Name Value, parent :: Env } data Value = Num Int | Clo Name Expr Env envLookup :: Name -> Env -> Value envLookup n Root = Num 10000 --- fix it envLookup n env = case lookup n (current env) of Just v -> v otherwise -> envLookup n (parent env) --------------- setvar :: Name -> Value -> Env -> Env setvar n v Root = Env{current=fromList [(n, v)] , parent=Root } setvar n v Env{current=c, parent=p} = Env{current=(insert n v c), parent=p } eval :: Expr -> Env -> (Value, Env) eval (Val n) e = (Num n , e) eval (Add [x , y]) e = case eval x e of (Num n, e') -> case eval y e of (Num m, e1') -> (Num (n+m), e) eval (Add [x]) e = case eval x e of (Num n, e') -> (Clo "y" (Add [(Val n), (Var "y")]) e' , e) eval (Var n) e = (envLookup n e, e) eval (Abs n x) e = (Clo n x e, e) eval (App x y) e = case eval x e of (Clo n' x' e', env') -> case eval y e of (vy, e'') -> (fst (eval x' (Env (fromList [(n', vy)]) e')), e ) --- the saved clojure env can't be changed eval (Define n e) env = case eval e env of (v, e') -> (Num 0, setvar n v env) --- fix me why 0 eval (Let (n, e1) e2) env = ------ Let should don't chanve the env (fst (eval e2 (Env (fromList [(n, (fst (eval e1 env)))]) env)), env) eval (Begin xs) e = case length xs of 0 -> (Num 0, Root) -- never come here 1 -> case eval (xs !! 0) e of (v, e') -> (v, e') n -> case eval (Begin (take (n - 1) xs)) e of (v, e') -> eval (last xs) e' test1 = eval (Begin [(Define "z" (Val 1)), (Define "y" (Abs "x" (Add [(Var "z"), (Var "x")]))), (Define "x" (Val 7)), (Define "z" (Val 90)), (Add [(Val 5), (App (Var "y") (Val 8))])]) Root test2 = eval (Begin [(Define "z" (Val 1)), (Define "y" (Abs "x" (Add [(Var "z"), (Var "x")]))), (Define "x" (Val 7)), (Let ("z", (Val 90)) (Add [(Val 5), (App (Var "y") (Val 8))])) ]) Root In case you want to see the total program. I put it into https://github.com/jiamo/correct_compiler/blob/master/src/Lambda_define.hs Here is my problem: I want test1 return (Num 103) I want test2 return (Num 14) But now it both return (Num 14) One problem I can figure out that "the eval of Define return a new env can't change the old env" Any one have suggests?

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latin file encoding
by Henning Thielemann 17 Nov '17

17 Nov '17

I have a LATIN-1 encoded file on a UTF-8 configured system: $ iconv -f latin1 Text Häßkell $ echo $LANG de_DE.UTF-8 Thus, a Haskell program cannot process it as it is: $ cat Text | ghc-8.2.2 -e 'interact id' | iconv -f latin1 <interactive>: <stdin>: hGetContents: invalid argument (invalid byte sequence) So far, so expected. Now I want to run it with latin encoding: $ cat Text | (LANG=de_DE ghc-8.2.2 -e 'interact id') | iconv -f latin1 <interactive>: <stdin>: hGetContents: invalid argument (invalid byte sequence) That's unexpected. Is this the reason: $ LANG=de_DE ghc-8.2.2 -e 'print System.IO.localeEncoding' ASCII ? Btw. what would be the Locale name for Latin-1 encoding?

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2017 state of Haskell survey results
by Taylor Fausak 16 Nov '17

16 Nov '17

I am excited to announce that the results of the 2017 state of Haskell survey are now available: http://taylor.fausak.me/2017/11/15/2017-state-of-haskell-survey-results/ <http://taylor.fausak.me/2017/11/15/2017-state-of-haskell-survey-results/> Thanks to everyone who responded!

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Dutch FP-day 2018 (early registration required)
by Arjen 16 Nov '17

16 Nov '17

I would like to invite you to the next Dutch Functional Programming Day. It will take place on Friday 5th of January 2018, at the Walter Bos Complex of the Dutch Tax and Customs Administration at John F. Kennedylaan 8, Apeldoorn. The Dutch Functional Programming Day is an annual gathering of researchers, students, and practitioners sharing a common interest in functional programming. Experts and newcomers to the field are equally welcome. For more information see http://wiki.clean.cs.ru.nl/NL-FP_dag_2018 Please note that to the nature of the location, registration before December 15 is required. We need a list of participants for the security staff. We also need this to order the right number of badges, drinks and pizza's. You can register by sending a mail to a.van.weelden(a)belastingdienst.nl (or reply to this e-mail) including: 1. your name (used for identification) 2. your affiliation, if any 3. whether you want (vegetarian) pizza 4. whether you want to give a talk There are no costs for anything this year, as it is fully sponsored by the Dutch Tax and Customs Administration. kind regards, Betsy Pepels and Arjen van Weelden

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Project idea, seeking feedback
by Alex 16 Nov '17

16 Nov '17

Hi all, I'm seeking feedback for a project I'd like to start. I have a bit of experience developing large scale systems containing many microservices, databases, message queues, and caches over many VMs. Time and time again I find myself confronted with the same problems: 1. It is difficult to trace events through the system: Consider an HTTP request made by a customer to a public API. Which microservices were impacted by that request? What SQL queries were run as a result of that request? What 3rd party APIs were consulted during the request's fulfillment? Answers to these questions are essential to fixing bugs quickly, and yet they are so difficult to answer (at least in my experience). 2. Problems are difficult to reproduce: When Customer Success walks in and says, "I have an angry customer on the phone. They want to know why [FOO] wasn't properly [BAR]" it is often impossible to give an answer without interactive troubleshooting and hours of grepping through unstructured log files. Troubleshooting may incur additional expenses too, since (for instance) you may hit your API request limit for a 3rd party service. 3. Business and non-business logic are not well encapsulated: Often I see code related to (for example) RabbitMQ interwoven with core business logic when calls need to be made to other microservices. The fact that RabbitMQ facilitates communication between microservices is an implementation detail that I shouldn't have to think about. 4. Resource consumption is non-uniform: Some microservices are more demanding than others in terms of CPU, memory, and disk usage. Achieving optimal "packing" is difficult. In other words, some VMs will have a high load and others will remain idle. Auto scaling groups can help with this in theory, but I don't think they can achieve the kind of density I would like to see. Moreover, what constitutes a "resource"? If a 3rd party service rate limits requests by IP address, couldn't each request be considered a resource unit which needs to be properly load balanced, just as you would with CPU? Given these motivations, I would like to flesh out some ideas for a framework/platform which addresses these issues. These ideas are half-baked and may not tie in well with one another. I envision a distributed system as follows: 1. One kind of VM: DevOps people have a saying: "Treat your VMs like cattle, not pets". In practice, "cattle" becomes "cows, chickens, pigs, and lobster". VMs typically have an assigned role, and they become part of a group which may or may not be auto-scaling. For a given instantiation of this hypothetical platform, I would like to see a single kind of VM. That is, every VM is identical to every other VM, and they all run the same Haskell application. 2. Strict separation of business and non-business logic: The framework should handle all aspects of communication between nodes (like Cloud Haskell does) in a pluggable and transparent way, but that's not all. The framework should have first class support for other integrations (such as PagerDuty alerting, performance monitoring, etc) which are described below. 3. Pool coordination via DSL: The entire pool of VMs is orchestrated/coordinated by one ore more "scripts" written in a DSL, which is implemented as a Free Monad. Every single "operation" or "primitive" in your AST data type is Serializable, and when the framework interprets the DSL, it serializes the instruction and sends it over the network to a node for execution. The particular node on which the instruction gets executed is chosen by the platform, not the developer. 4. Smart resource consumption: Each node brings with it a set of resources. It is *not* my intention to create a system which views CPU, memory, etc as a contiguous unit. Rather, each primitive instruction in the AST is viewed as a "black box" which can only consume as much CPU and memory as the node has available to it. The framework is responsible for profiling each instruction and scheduling future instructions to a node for which resources are predicted to be available. The developer should be able to define new resources such as 3rd party API calls, bandwidth, database connections, etc, all of which are profiled just as CPU and memory would. 5. Browser based control panel: Engineers should have a GUI at their disposal which allows them to watch -- in real time -- the execution flow of the DSL script. 6. Structured logs with advanced filtering: All log output should be structured with first class support for shipping the data to Logstash/ElasticSearch. The aforementioned GUI should be able to selectively filter output based on certain pre-defined predicates and display them to the developer. For example, if you're building an email virus scanning system (which may see millions of emails per day), you may want to limit the real-time debugging output to only a specific customer. 7. First class integration with modern tools and services: The system should integrate with Consul, PagerDuty, statsd, RabbitMQ, memcache, DataDog, Logstash, and Slack, with new integrations being easy to add. This is vital for clean separation of business and non-business logic. For example, the developer should be able to cache certain bits of data at will, without having to worry about opening and managing a TCP connection to memcache. This is my vision, and I want to build it completely in Haskell. What do you all think? -- Alex

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Re: [Haskell-cafe] how to make a KnownNat at runtime?
by dominic＠steinitz.org 16 Nov '17

16 Nov '17

You mean something like this? > withMatrix > :: forall z > . Matrix ℝ > -> (forall m n . (KnownNat m, KnownNat n) => L m n -> z) > -> z > withMatrix a f = > case someNatVal $ fromIntegral $ rows a of > Nothing -> error "static/dynamic mismatch" > Just (SomeNat (_ :: Proxy m)) -> > case someNatVal $ fromIntegral $ cols a of > Nothing -> error "static/dynamic mismatch" > Just (SomeNat (_ :: Proxy n)) -> > f (mkL a :: L m n) > Dear Cafe, > > I am using type-level numbers for > representing dimensions of vectors and matrices > (similar to https://github.com/ekmett/ersatz/blob/master/tests/Z001.hs ) > Now when I indeed know all the dimensions at compile time, > (in the example code, line 20) everything works nicely. > > But what do I do when the dimensions > will only become available at run time? > > Here is a simplified example: print the null vector > where the size is read from the command line - > of course in my application "handle" does more, > but it essentially has the type given here. > > I can extend the "case" in the "main" function > to include all the values I want to handle. > Looks ugly. Is there a better way? > > - J. > > {-# language KindSignatures, RankNTypes, LambdaCase, > TypeApplications, DataKinds, ScopedTypeVariables > #-} > > import GHC.TypeLits > import Data.Proxy > import System.Environment > > main :: IO () > main = getArgs >>= \ case > [ "2" ] -> handle (Proxy :: Proxy 2) > [ "3" ] -> handle (Proxy :: Proxy 3) > > handle :: forall (n::Nat) . KnownNat n => Proxy n -> IO () > handle (_ :: Proxy n) = print (zero :: V n Int) > > data V (n::Nat) a = V [a] deriving Show > > zero :: forall (n::Nat) a . (KnownNat n, Num a) => V n a > zero = V $ replicate (fromIntegral $ natVal (Proxy :: Proxy n)) 0 > Dominic Steinitz dominic(a)steinitz.org http://idontgetoutmuch.wordpress.com

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ANNOUNCE: Haskell Communities and Activities Report (33rd ed., November 2017)
by Mihai Maruseac 16 Nov '17

16 Nov '17

On behalf of all the contributors, we are pleased to announce that the Haskell Communities and Activities Report (33rd edition, November 2017) is now available, in PDF and HTML formats: http://haskell.org/communities/11-2017/report.pdf http://haskell.org/communities/11-2017/html/report.html All previous editions of HCAR can be accessed on the wiki at https://wiki.haskell.org/Haskell_Communities_and_Activities_Report Many thanks go to all the people that contributed to this report, both directly, by sending in descriptions, and indirectly, by doing all the interesting things that are reported. We hope you will find it as interesting a read as we did. If you have not encountered the Haskell Communities and Activities Reports before, you may like to know that the first of these reports was published in November 2001. Their goal is to improve the communication between the increasingly diverse groups, projects, and individuals working on, with, or inspired by Haskell. The idea behind these reports is simple: Every six months, a call goes out to all of you enjoying Haskell to contribute brief summaries of your own area of work. Many of you respond (eagerly, unprompted, and sometimes in time for the actual deadline) to the call. The editors collect all the contributions into a single report and feed that back to the community. When we try for the next update, six months from now, you might want to report on your own work, project, research area or group as well. So, please put the following into your diaries now: ======================================== End of February 2018: target deadline for contributions to the May 2018 edition of the HCAR Report ======================================== Unfortunately, many Haskellers working on interesting projects are so busy with their work that they seem to have lost the time to follow the Haskell related mailing lists and newsgroups, and have trouble even finding time to report on their work. If you are a member, user or friend of a project so burdened, please find someone willing to make time to report and ask them to "register" with the editors for a simple e-mail reminder in November (you could point us to them as well, and we can then politely ask if they want to contribute, but it might work better if you do the initial asking). Of course, they will still have to find the ten to fifteen minutes to draw up their report, but maybe we can increase our coverage of all that is going on in the community. Feel free to circulate this announcement further in order to reach people who might otherwise not see it. Enjoy! -- Mihai Maruseac (MM) "If you can't solve a problem, then there's an easier problem you can solve: find it." -- George Polya

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