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  • GHCi error: Not in scope

    - by dmindreader
    I'm trying to compile this function from Learn You a Haskell for Great Good. removeNonUppercase st = [ c | c <- st, c `elem` ['A'..'Z']] by placing it into a removeNonUpperCase.hs file. It compiles fine, but when passing the argument: ghci> removeNonUppercase "Hahaha! Ahahaha!" the compiler says: <interactive>:1:0: Not in scope: 'removeNonUpperCase' Why?

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  • How to define a function in ghci across multiple lines

    - by Peter McGrattan
    I'm trying to define any simple function that spans multiple lines in ghci, take the following as an example: let abs n | n >= 0 = n | otherwise = -n So far I've tried pressing Enter after the first line: Prelude> let abs n | n >= 0 = n Prelude> | otherwise = -n <interactive>:1:0: parse error on input `|' I've also attempted to use the :{ and :} commands but I don't get far: Prelude> :{ unknown command ':{' use :? for help. I'm using GHC Interactive version 6.6 for Haskell 98 on Linux, what am I missing?

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  • How do I use multiple where clauses in GHCi?

    - by T.R.
    I'm playing around with GHCi for the first time, and I'm having some trouble writing multi-line functions. My code is as follows: Prelude> :{ Prelude| let diffSquares lst = abs $ squareOfSums lst - sumOfSquares lst Prelude| where Prelude| squareOfSums lst = (fst (sumsAndSquares lst))^2 Prelude| sumOfSquares lst = snd (sumsAndSquares lst) Prelude| sumsAndSquares = foldl (\(sms,sqrs) x -> (sms+x,sqrs+x^2)) (0,0) Prelude| :} It gives the following error: <interactive>:1:142: parse error on input `=' Could someone kindly point me in the direction of what I'm missing?

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  • Why does ghci say that 1.1 + 1.1 + 1.1 > 3.3 is True?

    - by Frerich Raabe
    I've been going through a Haskell tutorial recently and noticed this behaviour when trying some simple Haskell expressions in the interactive ghci shell: Prelude> 1.1 + 1.1 == 2.2 True Prelude> 1.1 + 1.1 + 1.1 == 3.3 False Prelude> 1.1 + 1.1 + 1.1 > 3.3 True Prelude> 1.1 + 1.1 + 1.1 3.3000000000000003 Does anybody know why that is?

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  • Info on type family instances

    - by yairchu
    Intro: While checking out snoyman's "persistent" library I found myself wanting ghci's (or another tool) assistance in figuring out stuff. ghci's :info doesn't seem to work as nicely with type-families and data-families as it does with "plain" types: > :info Maybe data Maybe a = Nothing | Just a -- Defined in Data.Maybe ... > :info Persist.Key Potato -- "Key Potato" defined in example below data family Persist.Key val -- Defined in Database.Persist ... (no info on the structure/identity of the actual instance) One can always look for the instance in the source code, but sometimes it could be hard to find it and it may be hidden in template-haskell generated code etc. Code example: {-# LANGUAGE FlexibleInstances, GeneralizedNewtypeDeriving, MultiParamTypeClasses, TypeFamilies, QuasiQuotes #-} import qualified Database.Persist as Persist import Database.Persist.Sqlite as PSqlite PSqlite.persistSqlite [$persist| Potato name String isTasty Bool luckyNumber Int UniqueId name |] What's going on in the code example above is that Template-Haskell is generating code for us here. All the extensions above except for QuasiQuotes are required because the generated code uses them. I found out what Persist.Key Potato is by doing: -- test.hs: test = PSqlite.persistSqlite [$persist| ... -- ghci: > :l test.hs > import Language.Haskell.TH > import Data.List > runQ test >>= putStrLn . unlines . filter (isInfixOf "Key Potato") . lines . pprint where newtype Database.Persist.Key Potato = PotatoId Int64 type PotatoId = Database.Persist.Key Potato Question: Is there an easier way to get information on instances of type families and data families, using ghci or any other tool?

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  • Compiled Haskell libraries with FFI imports are invalid when imported into GHCI

    - by John Millikin
    I am using GHC 6.12.1, in Ubuntu 10.04 When I try to use the FFI syntax for static storage, only modules running in interpreted mode (ie GHCI) work properly. Compiled modules have invalid pointers, and do not work. I'd like to know whether anybody can reproduce the problem, whether this an error in my code or GHC, and (if the latter) whether it's a known issue. I'm using sys_siglist because it's present in a standard library on my system, but I don't believe the actual storage used matters (I discovered this while writing a binding to libidn). If it helps, sys_siglist is defined in <signal.h> as: extern __const char *__const sys_siglist[_NSIG]; I thought this type might be the problem, so I also tried wrapping it in a plain C procedure: #include<stdio.h> const char **test_ffi_import() { printf("C think sys_siglist = %X\n", sys_siglist); return sys_siglist; } However, importing that doesn't change the result, and the printf() call prints the same pointer value as show siglist_a. My suspicion is that it's something to do with static and dynamic library loading. Update: somebody in #haskell suggested this might be 64-bit specific; if anybody tries to reproduce it, can you mention your architecture and whether it worked in a comment? Code as follows: -- A.hs {-# LANGUAGE ForeignFunctionInterface #-} module A where import Foreign import Foreign.C foreign import ccall "&sys_siglist" siglist_a :: Ptr CString -- -- B.hs {-# LANGUAGE ForeignFunctionInterface #-} module B where import Foreign import Foreign.C foreign import ccall "&sys_siglist" siglist_b :: Ptr CString -- -- Main.hs {-# LANGUAGE ForeignFunctionInterface #-} module Main where import Foreign import Foreign.C import A import B foreign import ccall "&sys_siglist" siglist_main :: Ptr CString main = do putStrLn $ "siglist_a = " ++ show siglist_a putStrLn $ "siglist_b = " ++ show siglist_b putStrLn $ "siglist_main = " ++ show siglist_main peekSiglist "a " siglist_a peekSiglist "b " siglist_b peekSiglist "main" siglist_main peekSiglist name siglist = do ptr <- peekElemOff siglist 2 str <- maybePeek peekCString ptr putStrLn $ "siglist_" ++ name ++ "[2] = " ++ show str I would expect something like this output, where all pointer values identical and valid: $ runhaskell Main.hs siglist_a = 0x00007f53a948fe00 siglist_b = 0x00007f53a948fe00 siglist_main = 0x00007f53a948fe00 siglist_a [2] = Just "Interrupt" siglist_b [2] = Just "Interrupt" siglist_main[2] = Just "Interrupt" However, if I compile A.hs (with ghc -c A.hs), then the output changes to: $ runhaskell Main.hs siglist_a = 0x0000000040378918 siglist_b = 0x00007fe7c029ce00 siglist_main = 0x00007fe7c029ce00 siglist_a [2] = Nothing siglist_b [2] = Just "Interrupt" siglist_main[2] = Just "Interrupt"

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  • Unsure of how to get the right evaluation order

    - by Matt Fenwick
    I'm not sure what the difference between these two pieces of code is (with respect to x), but the first one completes: $ foldr (\x y -> if x == 4 then x else x + y) 0 [1,2 .. ] 10 and the second one doesn't (at least in GHCi): $ foldr (\x (y, n) -> if x == 4 then (x, n) else (x + y, n + 1)) (0, 0) [1,2 .. ] ....... What am I doing wrong that prevents the second example from completing when it hits x == 4, as in the first one? I've tried adding bang-patterns to both the x and to the x == 4 (inside a let) but neither seems to make a difference.

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  • Why I am forced to write the (Data Constructor) name with first letter in small case?

    - by Optimight
    Why I am forced to write "liOfLi" in place of "LiOfLi"? Please guide. code in baby.hs LiOfLi = [ [1,3,4,5,6,8], [ 12, 13, 15, 16, 19, 20], [23, 24, 25, 45, 56] ] ghci response: ghci :l baby [1 of 1] Compiling Main ( baby.hs, interpreted ) Failed, modules loaded: none. ghci baby.hs:29:1: Not in scope: data constructor `LiOfLi' When changing the initial letter to smaller case code in baby.hs liOfLi = [ [1,3,4,5,6,8], [ 12, 13, 15, 16, 19, 20], [23, 24, 25, 45, 56] ] ghci response: ghci :l baby [1 of 1] Compiling Main ( baby.hs, interpreted ) Ok, modules loaded: Main. Following are the SO questions I refered but I failed to understand the rules/ logic and get the answer for (my) abovementioned question. Why does Haskell force data constructor's first letter to be upper case? the variable names need to be lowercase. The official documentation related to this is at haskell.org/onlinereport/intro.html#namespaces – (the SO comment by) Chris Kuklewicz

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  • Beginning Haskell: "not in scope" Unprecedented error

    - by user1071838
    So I just started learning Haskell, and this (http://learnyouahaskell.com) nifty book is giving a lot of help. So yesterday I wrote in a text file doubleMe x = x + x and saved it as double.hs. So after saving that I open up my command prompt, CD to the right folder, type in "ghci" to get haskell started, and then type in >doubleMe 5 10 and everything seems to work. Now today, I do the same thing and this happens (actual copy paste from command line) . . . C:\Users\myName\haskell>ghci GHCi, version 7.0.3: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Loading package ffi-1.0 ... linking ... done. Prelude> :l double.hs [1 of 1] Compiling Main ( double.hs, interpreted ) Ok, modules loaded: Main. *Main> doubleMe 5 <interactive>:1:1: Not in scope: `doubleMe' So basically, everything was working fine, but now haskell can't find the function I wrote in double.hs. Can anyone tell what is going on? I'm pretty lost and confused. This is just a guess but does it have to do with *Main at all? Thanks for the help.

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  • How do you use kate? Tips/Tricks/Workflow

    - by Roman A. Taycher
    We all seen a bunch of these? Mostly for IDE's but also for vim and emacs. Kate is (only) a text editor (an awesome one) but it has a ton of options plus a number of plugins, so its hard to know all of it well. How do use the Kate text editor? Please share your workflow and help me and others learn some of the cool tricks you use. I'll start I use the built in terminal a lot opening files quickly, and using it as an enhanced haskell repl with ghci (since ghci doesn't allow you all to just put in all kinds of haskell code). Also use split views to quickly compare files (especially different versions of the same file). Also the auto-complete maybe simple(more use for saving typing time then remembering functions) but it works really well for that. Also if You highlight something and hit a start [/{/( it puts it in between brackets rather then replacing it with a bracket(why the hell do a lot of IDEs not have this feature).

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  • Instantiate type variable in Haskell

    - by danportin
    EDIT: Solved. I was unware that enabling a language extension in the source file did not enable the language extension in GHCi. The solution was to :set FlexibleContexts in GHCi. I recently discovered that type declarations in classes and instances in Haskell are Horn clauses. So I encoded the arithmetic operations from The Art of Prolog, Chapter 3, into Haskell. For instance: fac(0,s(0)). fac(s(N),F) :- fac(N,X), mult(s(N),X,F). class Fac x y | x -> y instance Fac Z (S Z) instance (Fac n x, Mult (S n) x f) => Fac (S n) f pow(s(X),0,0) :- nat(X). pow(0,s(X),s(0)) :- nat(X). pow(s(N),X,Y) :- pow(N,X,Z), mult(Z,X,Y). class Pow x y z | x y -> z instance (N n) => Pow (S n) Z Z instance (N n) => Pow Z (S n) (S Z) instance (Pow n x z, Mult z x y) => Pow (S n) x y In Prolog, values are insantiated for (logic) variable in a proof. However, I don't understand how to instantiate type variables in Haskell. That is, I don't understand what the Haskell equivalent of a Prolog query ?-f(X1,X2,...,Xn) is. I assume that :t undefined :: (f x1 x2 ... xn) => xi would cause Haskell to instantiate xi, but this gives a Non type-variable argument in the constraint error, even with FlexibleContexts enabled.

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  • Creating Haskell instance declarations

    - by btl
    Hello, complete noob to Haskell here with probably an even noobier question. I'm trying to get ghci output working and am stuck on instance declarations. How could I declare an instance for "(Show (Stack - Stack))" given: data Cmd = LD Int | ADD | MULT | DUP deriving Show type Prog = [Cmd] type Stack = [Int] type D = Stack -> Stack I've been trying to create a declaration like: instance Show D where show = Stack but all my attempts have resulted in illegal instance declarations. Any help and/or references much appreciated!

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  • Infinite loop in haskell? (newbie)

    - by Mike
    I'm just learning Haskell. I thought this would produce a factorial function... (within ghci) Prelude let ft 0 = 1 Prelude let ft n = n * ft (n - 1) Prelude ft 5 (hangs indefinitely, until ^C). Can someone point me in the right direction? Thanks!

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  • What is wrong with this list comprehension code?

    - by suresh
    My aim is to list all elements of the array a whose values are greater than their index positions. I wrote a Haskell code like this. [a|i<-[0..2],a<-[1..3],a!!i>i] When tested on ghci prelude prompt, I get the following error message which I am unable to understand. No instance for (Num [a]) arising from the literal 3 at <interactive>:1:20 Possible fix: add an instance declaration for (Num [a])

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  • Haskell: type inference and function composition

    - by Pillsy
    This question was inspired by this answer to another question, indicating that you can remove every occurrence of an element from a list using a function defined as: removeall = filter . (/=) Working it out with pencil and paper from the types of filter, (/=) and (.), the function has a type of removeall :: (Eq a) => a -> [a] -> [a] which is exactly what you'd expect based on its contract. However, with GHCi 6.6, I get gchi> :t removeall removeall :: Integer -> [Integer] -> [Integer] unless I specify the type explicitly (in which case it works fine). Why is Haskell inferring such a specific type for the function?

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  • Haskell simple compilation bug

    - by fmsf
    I'm trying to run this code: let coins = [50, 25, 10, 5, 2,1] let candidate = 11 calculate :: [Int] calculate = [ calculate (x+candidate) | x <- coins, x > candidate] I've read some tutorials, and it worked out ok. I'm trying to solve some small problems to give-me a feel of the language. But I'm stuck at this. test.hs:3:0: parse error (possibly incorrect indentation) Can anyone tell me why? I've started with haskell today so please go easy on the explanations. I've tried to run it like: runghc test.hs ghc test.hs but with: ghci < test.hs it gives this one: <interactive>:1:10: parse error on input `=' Thanks

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  • (type theoretical) How is ([] ==) [] typed in haskell?

    - by Ingo
    It sounds silly, but I can't get it. Why can the expression [] == [] be typed at all? More specifically, which type (in class Eq) is inferred to the type of list elements? In a ghci session, I see the following: Prelude> :t (==[]) (==[]) :: (Eq [a]) => [a] -> Bool But the constraint Eq [a] implies Eq a also, as is shown here: Prelude> (==[]) ([]::[IO ()]) <interactive>:1:1: No instance for (Eq (IO ())) arising from use of `==' at <interactive>:1:1-2 Probable fix: add an instance declaration for (Eq (IO ())) In the definition of `it': it = (== []) ([] :: [IO ()]) Thus, in []==[], the type checker must assume that the list element is some type a that is in class Eq. But which one? The type of [] is just [a], and this is certainly more general than Eq a = [a].

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  • Haskell: What is the difference between $ (dollar) and $! (dollar exclamation point)

    - by Jelle Fresen
    Can anybody explain the difference in Haskell between the operators ($) and ($!) (dollar sign vs dollar sign exclamation point)? I haven't seen the use of $! anywhere so far, but while browsing through the Haskell reference on www.zvon.org, I noticed its existence and that it has the exact same definition as $. When trying some simple statements in a Haskell interpreter (ghci), I couldn't find any difference, nor could I find any reference to the operator in the top listed tutorials when googling for haskell tutorial. So, just out of curiosity, what is the difference, if at all?

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  • Better exception for non-exhaustive patterns in case

    - by toofarsideways
    Is there a way to get GHCi to produce better exception messages when it finds at runtime that a call has produced value that does not match the function's pattern matching? It currently gives the line numbers of the function which produced the non-exhaustive pattern match which though helpful at times does require a round of debugging which at times I feel is doing the same set of things over and over. So before I tried to put together a solution I wanted to see if something else exists. An exception message that in addition to giving the line numbers shows what kind of call it attempted to make? Is this even possible?

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  • Problem with do construct in haskell

    - by Anonymous Coward
    Hello everyone I'm trying to learn Haskell and want to write a small program which prints the content of a file to the screen. When I load it into GHCi I get the following error: The last statement in a 'do' construct must be an expression I know this question has be asked already here: Haskell — “The last statement in a 'do' construct must be an expression”. Even though my code is very similar I still can't figure out the problem. If anyone could point out the problem to me I'd be very thankful. module Main (main) where import System.IO import System(getArgs) main :: IO() main = do args <- getArgs inh <- openFile $ ReadMode head args printFile inh hClose inh printFile :: Handle -> IO () printFile handle = do end <- hIsEOF handle if end then return () else do line <- hGetLine handle putStrLn line printFile handle

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