Tag: haskell
Starting from a simple case of "fold" (I used (+) but can be anything else):
Prelude.foldl (+) 0 [10,20,30]
is it possible apply an inline transformation similar to (that doesn't work):
Prelude.foldl ((+) . (\x -> read x :: Int)) 0 ["10","20","30"]
In case not, is there an alternative to fold, to apply a generic function and inline transformation (apart from using specific functions like 'sum', 'max' etc)?
The read lambda applies to the first argument and the first argument to the function given to foldl is the accumulator. Those two arguments are the opposite for foldr. So, expanded, it looks like this:
foldl (\acc element -> (read acc :: Int) + element) 0 ["10", "20", "30"]
Since acc is an Int, this doesn't work.
So, with this information in hand, you can do this with foldr since it has the opposite argument order:
foldr ((+) . (read :: String -> Int)) 0 ["10","20","30"]
If you want an inline foldl version, you can use flip to achieve this.
You could also use map first (everything else being equal, this would be the solution I'd prefer):
foldl (+) 0 $ map (read :: String -> Int) ["10","20","30"]
sockets,haskell,network-programming,io-monad
The idiomatic way to repeat the same action over and over again forever is forever serverLoop :: Socket -> IO () serverLoop sock = forever $ do (conn, _) <- accept sock forkIO $ handleConn conn ...
shell,haskell,command-line-arguments,executable
The documentation for readProcess says: readProcess :: FilePath Filename of the executable (see RawCommand for details) -> [String] any arguments -> String standard input -> IO String stdout When it's asking for standard input it's not asking for a file to read the input from, but the actual contents of...
For the Not in scope: data constructor 'Integer' part, the problem is that you have an extra Integer in the line isDigit c = TNumber Integer (read c) : tokenize cs which should be isDigit c = TNumber (read [c]) : tokenize cs The [c] part is needed because read...
haskell,type-level-computation,hlist
Answering your comment: Actually, I can do if I can filter the heterogeneous list by type. Is that possible? You can filter the heterogeneous list by type if you add a Typeable constraint to b. The main idea is we will use Data.Typeable's cast :: (Typeable a, Typeable b) =>...
I assume that we'd like to have a solution for the general case where the changing type parameter is not necessarily in the right position for DeriveFunctor. We can distinguish two cases. In the simple case out data type is not recursive. Here, prisms are a fitting solution: {-# LANGUAGE...
Here is a simple example (@luqui mentioned) you should be able to generalize to your need: module Main where import Control.Monad (replicateM) import System.Random (randomRIO) main :: IO () main = do randomList <- randomInts 10 (1,6) print randomList let s = myFunUsingRandomList randomList print s myFunUsingRandomList :: [Int] ->...
<**> from Control.Applicative is flip <*>. Your example can work with that, slightly rearranged: >((+) <$> Just 3 <*> Just 5) <**> ((+) <$> Just 6) Just 14 ...
For some reason, cabal wasn't using the version I thought it was (1.5) but (1.4) probably from the haskell platform. Uprading fixed the problem.
You can certainly do this without changing the type signature of func :: [Int] -> [Int]: have func call a different function, which takes an extra argument that is the counter you were talking about: func :: [Int] -> [Int] func = go 0 where go _ [] = []...
haskell,concurrency,network-programming
Three days later and its solved: Was actually unrelated to either the networking or concurrency code, and infact caused by my incorrect re-implementation of Yampas dpSwitch in Netwire. Corrected code posted below for anyone wishing to implement this function: dpSwitch :: (Monoid e, Applicative m, Monad m, T.Traversable col) =>...
ubuntu,haskell,vim,ubuntu-14.04,cabal
Your $PATH variable seems to be broken. In a comment you said it was /home/me/google-cloud-sdk/bin:/.cabal/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games This means that your shell (assumed to be bash) will look in the following directories /home/me/google-cloud-sdk/bin /.cabal/bin /usr/local/sbin /usr/local/bin /usr/sbin /usr/bin /sbin /bin /usr/games /usr/local/games when looking for executable. If you look at the second...
All you need is love and to split print into putStrLn . show and then add a simple map in-between which does the conversion: main :: IO () main = let fn '[' = '{' fn ']' = '}' fn c = c in (readLn :: IO [Integer]) >>= putStrLn...
This cannot be done currently in diagrams, although it is something we would like to have in the future. You can get part of the way there using the diagrams-canvas backend, but that only displays on a local host and cannot be embedded into a web page. The only thing...
You can use the same applicative notation to parse the nested values like this: instance FromJSON DataMain where parseJSON (Object v) = DataMain <$> v .: "a" <*> v .: "b" <*> (Data1 <$> v .: "c" <*> v .: "d") parseJSON _ = mzero ...
linux,haskell,make,ghc,theorem-proving
Looks like paradox was written for a rather old version of GHC. You can fix all these "Could not find module" errors by using GHC version 7.8 or older and setting GHC = ghc -hide-package base -package haskell98 in the Makefile, though you will likely encounter more errors after that....
haskell,file-io,lazy-evaluation
You're right, this is a pain. Avoid using the old standard file IO module, for this reason – except to simply read an entire file that won't change, as you did; this can be done just fine with readFile. readCsvContents :: Filepath -> IO String readCsvContents fileName = do contents...
I am not ready to give a nice list of best practises, but for starters if you want to keep stuff sanely organized, use explicit exports instead of just exporting everything, e.g: module Parser ( parseConfig ) where ... Explicit exports also allow you to reexport your imports, e.g. module...
haskell,syntax,infix-notation,applicative,infix-operator
Why is f <$> g <$> x equivalent to (f . g) <$> x ...well, this isn't so much a functor-thing as a Haskell-thing. The reason it works is that functions are functors. Both <$> operators work in different functors! f <$> g is in fact the same as...
You may write: main = readLn >>= print . subsequences You will need to nail down the type to be read, for example by having a monomorphic subsequences or by annotating readLn. In ghci: Data.List> (readLn :: IO [Integer]) >>= print . subsequences [1,2,3] [[],[1],[2],[1,2],[3],[1,3],[2,3],[1,2,3]] (I typed in the first...
Simple decimal literals without type indicator (i.e. one of the letters lndf) do not automatically have type Int in Frege. They will get assigned the type you probably wanted, and the literal will get adapted accordingly. This is why they are called DWIM (do what I mean) literals. This is...
This is intentional: The UI.checkedChange event only triggers when the user clicks the checkbox, but not when it is set programmatically. The intention is that the bBool behavior represents the canonical state of the checkbox and the UI.checkedChange event represents request from the user to change it, which may or...
There are many approaches to this, mostly depending on what flavor of devops/hosting your prefer. One option is to put a dummy value in the config file and override it with an environment variable at runtime (see: https://github.com/yesodweb/yesod/wiki/Configuration#overriding-configuration-values-with-environment-variables). You can also having an extra settings file for production that overrides...
Using multi-ghc-travis, you can also set up Travis-CI for ghc 7.10 (apart from other versions).
haskell,types,binding,dependent-type
The type you suggest can not be implemented, i.e. it is not inhabited: takeWhileVector :: (a -> Bool) -> Vector a n -> Vector a m Remember that the caller chooses the type variables a,n,m. This means that the caller can use your function as e.g. takeWhileVector :: (a ->...
Here is a function f' which does what you describe. f' :: [(String,String)] -> IO [Bool] f' = mapM $ uncurry f Let me know if something is unclear! And, just to be clear, here is how you run it: main = do res <- f' [("a.txt", "b.txt"), ("c.txt", "d.txt")]...
the problem is main = ... main should have type IO () but you give an expression with type [[Integer]] (as the compiler tells you) - so as I think you want to output the result to the console I think you are looking for print this works for me:...
string,function,haskell,recursion,parameters
Yes, once you call again f with a new value of n, it has no way to reference the old value of n unless you pass it explicitly. One way to do it is to have an internal recursive function with its width parameter, as you have, but that can...
list,haskell,functional-programming,idiomatic
The multiple call to addPoints could be replaced by a fold. As suggested in a comment, reversing your addPoint function would make things easier: addPoint' :: Point -> Polyline -> Polyline addPoint' p line = p:line So then your constructLine function could build a temporary list of the points to...
Here's one that I wrote a few weeks ago. You can specify the number of decimals you want (correctly rounded), or just pass Nothing in which case it will print the full precision, including marking the repeated decimals. module ShowRational where import Data.List(findIndex, splitAt) -- | Convert a 'Rational' to...
haskell,compiler-errors,instance,equality,typeclass
It isn't clear what you are trying to achieve. Here are some thoughts: When you declare an instance of a class like instance (Eq a) => PartOrd a, you are expected to provide implementations for the functions in PartOrd a (ie partcmp, not == and \=). The compiler is telling...
using TypeFamilies The problem is that you somehow have to connect b with your collection (the elements in it) - there are several ways to do this but I think a rather nice one is using TypeFamilies: {-# LANGUAGE TypeFamilies #-} module Test where import qualified Data.Map as Map import...
createNotificationIdentifierItem :: APNSIdentifier -> APNSItem createNotificationIdentifierItem (Identifier identifier) = Item $ do putWord8 3 putWord16be 4 putWord32be identifier or createNotificationIdentifierItem :: APNSIdentifier -> APNSItem createNotificationIdentifierItem (Identifier identifier) = do Item $ putWord8 3 Item $ putWord16be 4 Item $ putWord32be identifier after making APNSItem an instance of Monad (you can...
The code you posted desugars into the following. x >>= (\a -> print a >> return 500) Or, expanding out the definition of (>>) x >>= (\a -> print a >>= (\_ -> return 500)) Then, you can see that in the different calls to (>>=), the types a and...
Well, foo (x:y:z:xs) plus a “too short clause” certainly wouldn't be a bad solution. Another would be foo xs = case splitAt 3 xs of ([x,y,z],xs') -> calc x y z : foo (y:z:xs') _ -> [] Or, perhaps nicest, import Data.List (tails) foo xs = [ calc x y...
haskell,functional-programming,runtime,transactional-memory
(# s2#, TVar tvar# #) is an unboxed tuple. The name stg_newTVarzh is built from: The stg_ prefix, which is common to the whole GHC runtime, and stands for the spineless-tagless G-machine, an abstract machine to evaluate functional languages; newTVar which is the first part of newTVar#; the final zh,...
haskell,lambda,instance,show,typeclass
Add an instance declaration for the Show class. instance Show LExpr where show = show' And remove the deriving(Show) part data LExpr = Variable String -- variable | Apply LExpr LExpr -- function application | Lambda String LExpr -- Lambda abstraction deriving (Eq) ...
length is O(1), so splitAt suffices to define everything you need, in an efficient way. splitAtR i s = splitAt (length s - i) s takeR i s = snd $ splitAtR i s dropR i s = fst $ splitAtR i s According to the docs, splitAt costs O(log(min(i,length...
Depending on if consuming the whole input should be the property of parseNoteDocument or just the tests, I'd extend one or the other with endOfInput or atEnd. I'd suggest to define a proper Parser for your documents, like parseNoteDocument' :: Text -> Parsec NoteDocument parseNoteDocument' = many parseLine and then...
take is of type Int -> [a] -> [a], i.e. it returns a list. It seems you’re looking for head, which returns one element. head $ head $ repeat [1..] ...
Consider the simpler problem of summing the first 100 positive integers: sum [x | x <- [1,2..], x <= 100] This doesn't work either. As a human, you know that once x <= 100 returns False, it will never return True again, because x is getting larger. But Haskell doesn't...
string,function,haskell,if-statement,recursion
Your code doesn't handle the case where a line is shorter than the maximum length. This is somewhat obscured by another bug: n is decremented until a whitespace is found, and then f is called recursively passing this decremented value of n, effectively limiting all subsequent lines to the length...
haskell,cabal,cabal-install,nix,haskell-ng
Could someone please explain what haskellng is in a simple, clear way? Well, haskellng is the next generation Nix Haskell package set made for Nix. I think most of the work was done by Peter Simons. But note that in the latest master version, haskellngPackages has been renamed back...
Tying the not like that doesn't appear to increase sharing. Contrast with: cycle xs = let x = xs ++ x in x Tying the knot here has the effect of creating a circular linked list in memory. x is its own tail. There's a real gain. Your suggested implementation...
Here's an example which uses random-fu: import Data.Random -- for randomness import Text.Printf -- for printf import Data.Foldable -- for the for_ loop -- pdf and cdf are basically “Distribution -> Double -> Double” main = do -- defining normal distribution with mean = 10 and variation = 2 let...
haskell,types,monoids,type-variables,foldable
I found that this typechecks: {-# LANGUAGE RankNTypes #-} module FoldableTA where import Control.Category import Prelude hiding (id, (.)) class FoldableTA fm where foldMapTA :: Category h => (forall b c . a b c -> h b c) -> fm a b d -> h b d foldrTA ::...
You're making eval a bit too low-level. By including Literals in the signature. A better way to do this is, is using recursion: eval :: Expression -> Bool eval (Literal x) = x eval (Operation AND x y) = (eval x) && (eval y) eval (Operation OR x y) =...
The read lambda applies to the first argument and the first argument to the function given to foldl is the accumulator. Those two arguments are the opposite for foldr. So, expanded, it looks like this: foldl (\acc element -> (read acc :: Int) + element) 0 ["10", "20", "30"] Since...
A do block is for a specific type of monad, you can't just change the type in the middle. You can either transform the action or you can nest it inside the do. Most times transformations will be ready for you. You can, for instance have a nested do that...
Try to write your last line as def map(tree:Tree[Int])(f:Int=>Int) : Tree[Int] = fold(tree , EmptyTree:Tree[Int])((l,x,r) => Node(f(x),l,r)) Scala's type inference is very limited compared to haskell, in this case it tries to infere type of fold from it's arguments left to right, and incorectly decides that result type of fold...
This looks like a special case of a (jargon here but it can help with googling) paramorphism, a generalisation of primitive recursion to all initial algebras. Reimplementing ListCase Let's have a look at how to reimplement your function using such a combinator. First we define the notion of paramorphism: a...