July 2018 - Beginners

Best way to trig an IO action on a condition.
by PICCA Frederic-Emmanuel 29 Jul '18

29 Jul '18

Hello, Now that I know how to fork a process the right way :) I would like to trig this process when a list of file exist on the system. I can not use inotify since my files are on an nfs sytem. So I would like your advices in order to create a Condition which wait for the presence of these files and when this condition is fullliled, trigg the action. Thanks for your help. Frederic.

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Where is the accumulator in the expression foldr (<=<) return (replicate x oveKnight)? -- from LYAH example
by Olumide 26 Jul '18

26 Jul '18

Dear List, Chapter 13 of LYAH (http://learnyouahaskell.com/for-a-few-monads-more#useful-monadic-functions) has the following code block import Data.List inMany :: Int -> KnightPos -> [KnightPos] inMany x start = return start >>= foldr (<=<) return (replicate x oveKnight) What I'd like to know is where the accumulator of foldr is in this example. Regards, - Olumide

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forkProcess behaviour
by PICCA Frederic-Emmanuel 25 Jul '18

25 Jul '18

Hello, I try to write a service which execute an IO using forkProcess This service contain a web server whcih avwait for job published by users. for each job, I create a forkProcess of this job. I need to fork the process in order to change the uid and gid of the process for each of the job dependencing on who request the job. I have a least two questions 1) is this forkProcess a fork of all the current process, or is it just an executin of the IO in another process id ? I ask this because sometime one of my job hang and a process keep running. Since the original process is binded to a port, it is not possible to restart the server, saying that the port is already in use. 2 ) How can I catch the exceptions thrown from the child process in order to process them in the parent process. thanks for your help answering these questions. Frederic

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The type class Read
by mrx 13 Jul '18

13 Jul '18

Hi, I'm trying to make sense of the type class `Read`. It feels to be like a function performing casting to me, similar to what you have in C and Java. The weird thing is that the casting doesn't have to make sense, type-wise. So `read "5" :: Int` produce the integer 5 which make sense but `read "(3,'a')" :: Int` doesn't make sense to me but ghci accepts it! This is seeing Read as a function, as it's a type class i suspect I'm missing something. Quite possibly quite a lot... Why is the type class called `Read`? What am I missing above? // Patrik Iselind

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Two small questions from the section "Making a safe RPN calculator" in LYAH
by Olumide 11 Jul '18

11 Jul '18

Dear List, Question 1: In the section "Making a safe RPN calculator" of LYAH (Chapter 14 -- http://learnyouahaskell.com/for-a-few-monads-more#useful-monadic-functions) there is an expression: read numberString:xs, take from the last line of the function foldingFunction :: [Double] -> String -> [Double] foldingFunction (x:y:ys) "*" = (x * y):ys foldingFunction (x:y:ys) "+" = (x + y):ys foldingFunction (x:y:ys) "-" = (y - x):ys foldingFunction xs numberString = read numberString:xs My first question is why is the read function called before the cons operator? Question 2: The same section of the book also introduces the reads function which is used to implement the readMaybe function and a refactored foldingFunction readMaybe :: (Read a) => String -> Maybe a readMaybe st = case reads st of [(x,"")] -> Just x _ -> Nothing foldingFunction :: [Double] -> String -> Maybe [Double] foldingFunction (x:y:ys) "*" = return ((x * y):ys) foldingFunction (x:y:ys) "+" = return ((x + y):ys) foldingFunction (x:y:ys) "-" = return ((y - x):ys) foldingFunction xs numberString = liftM (:xs) (readMaybe numberString) I'd like to know why the foldingFunction returns Nothing in the following example: ghci> foldingFunction [] "1 wawawawa" Nothing Considering that reads "1 wawawawa" does not return Nothing, as follows ghci> reads "1 wawawawa" :: [(Double,String)] [(1.0," wawawawa")] Regards, - Olumide

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Haskell for Imperative Programmers
by Olivier Revollat 10 Jul '18

10 Jul '18

Hi, I've been using imperative languages for 20 years now :) I'm a beginner in haskell and I love the paradigm shift you feel when you come from imperative programming. I found interesting articles like : https://wiki.haskell.org/Haskell_IO_for_Imperative_Programmers Do you have any other ressources like that ? I'm not looking for how to use haskell in imperative style (e.g. with "do" notation, ...) no no ! I'm looking articles who explain how NOT TO USE imperative style with haskell, and help thinking the paradigm shift ... Thanks :)

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Storing the time difference between two Monotonic time results
by Awsaf Rahman 09 Jul '18

09 Jul '18

Hello, I am trying to time a function I have written in haskell using the clock package in the following way: *start <- getTime Monotonic* *evaluate(something)* *end <- getTime Monotonic* *fprint (timeSpecs % "\n") start end* Now what I want is to store the time difference between *start* and *end. *Is there a way I can do that? Regards Awsaf

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Where to report an unfriendly feature (if not a bug) in `stack ghci`?
by Jeffrey Brown 09 Jul '18

09 Jul '18

I don't know whether this is a problem with stack or with GHCI. But suppose you've got a bunch of modules loaded, and the prompt is hard to read, so you set it to something simpler like "> ". If you then use ":{" to enter a multi-line block of code, the module names appear again at the prompt, making the block you're entering very difficult to read. Here's an example: ) Ok, 55 modules loaded. Loaded GHCi configuration from /tmp/haskell-stack-ghci/d770186e/ghci-script *Vivid.UGens.UserInteraction Vivid Vivid.Actions Vivid.Actions.Class Vivid.Acti ons.IO Vivid.Actions.NRT Vivid.Actions.Scheduled Vivid.ByteBeat Vivid.Envelopes Vivid.NoPlugins Vivid.OSC Vivid.OSC.Util Vivid.Randomness Vivid.SCServer Vivid .SCServer.Connection Vivid.SCServer.State Vivid.SCServer.Types Vivid.SynthDef V ivid.SynthDef.FromUA Vivid.SynthDef.Literally Vivid.SynthDef.ToSig Vivid.SynthD ef.Types Vivid.SynthDef.TypesafeArgs Vivid.UGens Vivid.UGens.Algebraic Vivid.UG ens.Analysis Vivid.UGens.Args Vivid.UGens.Buffer Vivid.UGens.Conversion Vivid.U Gens.Convolution Vivid.UGens.Delays Vivid.UGens.Demand Vivid.UGens.Dynamics Viv id.UGens.Envelopes Vivid.UGens.Examples Vivid.UGens.FFT Vivid.UGens.Filters Viv id.UGens.Filters.BEQSuite Vivid.UGens.Filters.Linear Vivid.UGens.Filters.Nonlin ear Vivid.UGens.Filters.Pitch Vivid.UGens.Generators.Chaotic Vivid.UGens.Genera tors.Deterministic Vivid.UGens.Generators.Granular Vivid.UGens.Generators.Singl eValue Vivid.UGens.Generators.Stochastic Vivid.UGens.InOut Vivid.UGens.Info Viv id.UGens.Maths Vivid.UGens.Multichannel Vivid.UGens.Random Vivid.UGens.Reverbs Vivid.UGens.SynthControl Vivid.UGens.Triggers Vivid.UGens.Undocumented Vivid.UG ens.UserInteraction> :set prompt "> " > 3+1 4 > :{ *Vivid.UGens.UserInteraction Vivid Vivid.Actions Vivid.Actions.Class Vivid.Acti ons.IO Vivid.Actions.NRT Vivid.Actions.Scheduled Vivid.ByteBeat Vivid.Envelopes Vivid.NoPlugins Vivid.OSC Vivid.OSC.Util Vivid.Randomness Vivid.SCServer Vivid .SCServer.Connection Vivid.SCServer.State Vivid.SCServer.Types Vivid.SynthDef V ivid.SynthDef.FromUA Vivid.SynthDef.Literally Vivid.SynthDef.ToSig Vivid.SynthD ef.Types Vivid.SynthDef.TypesafeArgs Vivid.UGens Vivid.UGens.Algebraic Vivid.UG ens.Analysis Vivid.UGens.Args Vivid.UGens.Buffer Vivid.UGens.Conversion Vivid.U Gens.Convolution Vivid.UGens.Delays Vivid.UGens.Demand Vivid.UGens.Dynamics Viv id.UGens.Envelopes Vivid.UGens.Examples Vivid.UGens.FFT Vivid.UGens.Filters Viv id.UGens.Filters.BEQSuite Vivid.UGens.Filters.Linear Vivid.UGens.Filters.Nonlin ear Vivid.UGens.Filters.Pitch Vivid.UGens.Generators.Chaotic Vivid.UGens.Genera tors.Deterministic Vivid.UGens.Generators.Granular Vivid.UGens.Generators.Singl eValue Vivid.UGens.Generators.Stochastic Vivid.UGens.InOut Vivid.UGens.Info Viv id.UGens.Maths Vivid.UGens.Multichannel Vivid.UGens.Random Vivid.UGens.Reverbs Vivid.UGens.SynthControl Vivid.UGens.Triggers Vivid.UGens.Undocumented Vivid.UG ens.UserInteraction| f = let y = 3 *Vivid.UGens.UserInteraction Vivid Vivid.Actions Vivid.Actions.Class Vivid.Acti ons.IO Vivid.Actions.NRT Vivid.Actions.Scheduled Vivid.ByteBeat Vivid.Envelopes Vivid.NoPlugins Vivid.OSC Vivid.OSC.Util Vivid.Randomness Vivid.SCServer Vivid .SCServer.Connection Vivid.SCServer.State Vivid.SCServer.Types Vivid.SynthDef V ivid.SynthDef.FromUA Vivid.SynthDef.Literally Vivid.SynthDef.ToSig Vivid.SynthD ef.Types Vivid.SynthDef.TypesafeArgs Vivid.UGens Vivid.UGens.Algebraic Vivid.UG ens.Analysis Vivid.UGens.Args Vivid.UGens.Buffer Vivid.UGens.Conversion Vivid.U Gens.Convolution Vivid.UGens.Delays Vivid.UGens.Demand Vivid.UGens.Dynamics Viv id.UGens.Envelopes Vivid.UGens.Examples Vivid.UGens.FFT Vivid.UGens.Filters Viv id.UGens.Filters.BEQSuite Vivid.UGens.Filters.Linear Vivid.UGens.Filters.Nonlin ear Vivid.UGens.Filters.Pitch Vivid.UGens.Generators.Chaotic Vivid.UGens.Genera tors.Deterministic Vivid.UGens.Generators.Granular Vivid.UGens.Generators.Singl eValue Vivid.UGens.Generators.Stochastic Vivid.UGens.InOut Vivid.UGens.Info Viv id.UGens.Maths Vivid.UGens.Multichannel Vivid.UGens.Random Vivid.UGens.Reverbs Vivid.UGens.SynthControl Vivid.UGens.Triggers Vivid.UGens.Undocumented Vivid.UG ens.UserInteraction| in y + 1 *Vivid.UGens.UserInteraction Vivid Vivid.Actions Vivid.Actions.Class Vivid.Acti ons.IO Vivid.Actions.NRT Vivid.Actions.Scheduled Vivid.ByteBeat Vivid.Envelopes Vivid.NoPlugins Vivid.OSC Vivid.OSC.Util Vivid.Randomness Vivid.SCServer Vivid .SCServer.Connection Vivid.SCServer.State Vivid.SCServer.Types Vivid.SynthDef V ivid.SynthDef.FromUA Vivid.SynthDef.Literally Vivid.SynthDef.ToSig Vivid.SynthD ef.Types Vivid.SynthDef.TypesafeArgs Vivid.UGens Vivid.UGens.Algebraic Vivid.UG ens.Analysis Vivid.UGens.Args Vivid.UGens.Buffer Vivid.UGens.Conversion Vivid.U Gens.Convolution Vivid.UGens.Delays Vivid.UGens.Demand Vivid.UGens.Dynamics Viv id.UGens.Envelopes Vivid.UGens.Examples Vivid.UGens.FFT Vivid.UGens.Filters Viv id.UGens.Filters.BEQSuite Vivid.UGens.Filters.Linear Vivid.UGens.Filters.Nonlin ear Vivid.UGens.Filters.Pitch Vivid.UGens.Generators.Chaotic Vivid.UGens.Genera tors.Deterministic Vivid.UGens.Generators.Granular Vivid.UGens.Generators.Singl eValue Vivid.UGens.Generators.Stochastic Vivid.UGens.InOut Vivid.UGens.Info Viv id.UGens.Maths Vivid.UGens.Multichannel Vivid.UGens.Random Vivid.UGens.Reverbs Vivid.UGens.SynthControl Vivid.UGens.Triggers Vivid.UGens.Undocumented Vivid.UG ens.UserInteraction| :} > f 4 > -- Jeff Brown | Jeffrey Benjamin Brown Website <https://msu.edu/~brown202/> | Facebook <https://www.facebook.com/mejeff.younotjeff> | LinkedIn <https://www.linkedin.com/in/jeffreybenjaminbrown>(spammy, so I often miss messages here) | Github <https://github.com/jeffreybenjaminbrown>

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How does forever works ?
by Olivier Revollat 09 Jul '18

09 Jul '18

>From what I understand, this code main = forever $ do putStrLn "OK !" is equivalent to this one : main = do putStrLn "OK !" main In the second case, it's a simple recursion, so far so good ... but when I look at the implementation of "forever" i can't wrap my head around : http://hackage.haskell.org/package/base-4.11.1.0/docs/src/Control.Monad.htm… forever <http://hackage.haskell.org/package/base-4.11.1.0/docs/src/Control.Monad.htm…> a <http://hackage.haskell.org/package/base-4.11.1.0/docs/src/Control.Monad.htm…> = let a' <http://hackage.haskell.org/package/base-4.11.1.0/docs/src/Control.Monad.htm…> = a <http://hackage.haskell.org/package/base-4.11.1.0/docs/src/Control.Monad.htm…> *> <http://hackage.haskell.org/package/base-4.11.1.0/docs/src/GHC.Base.html#%2A…> a' <http://hackage.haskell.org/package/base-4.11.1.0/docs/src/Control.Monad.htm…> in a' <http://hackage.haskell.org/package/base-4.11.1.0/docs/src/Control.Monad.htm…> How does this works ? How does this make an infinite loop ? I understand that *> discard his right argument but it doesn't help me understand how forever implement an infinite loop ...

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Passing a file containing a list as an input
by Awsaf Rahman 06 Jul '18

06 Jul '18

Hello everyone! I am really struggling with Haskell I/O. I have written a mergesort program and as an input to that program I want to use a file containing a list of 1000 integers. Let's say the contents of the file look like this [120, 400, 500 , 20, 100 ..] How can I achieve that? I am trying to write a main function that takes the file as an input and passes the list to my mergesort function.. Regards Awsaf Rahman

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