There are two problems with your code: You are calling unsafePerformIO, but explicitly violating the contract of that function. It is on you to prove that the thing you provide to unsafePerformIO is actually pure, and the compiler is within its rights to act as if that’s the case, and here it is definitely not. … Read more
Wadler, School of Haskell, LYAH, HaskellWiki, Quora and many more describe the list monad. Compare: (=<<) :: Monad m => (a -> m b) -> m a -> m b for lists with concatMap :: (a -> [b]) -> [a] -> [b] for m = []. The regular (>>=) bind operator has the arguments flipped, … Read more
unsafePerformIO :: IO a -> a in System.IO.Unsafe (base). Use it with caution and read the description in the documentation carefully.
xs’ would be bound to the string “ello”. xs would be bound to the string “hello”. The @ pattern lets you give a name to a variable while also matching its structure and possibly giving name to the components.
Haskell’s grammar doesn’t allow you to use – like that. Use the subtract function instead: (subtract 3) 2
You could use tails: gather n l = filter ((== n) . length) $ map (take n) $ tails l or using takeWhile instead of filter: gather n l = takeWhile ((== n) . length) $ map (take n) $ tails l EDIT: You can remove the filter step by dropping the last n elements … Read more
You need a “unique supply”. The usual way to do this in Haskell is by threading a counter through the State monad, which automates the plumbing problem you describe.
What about: [ (x,y) | (x:rest) <- tails xs , y <- rest ]
As a general rule you write something like this: do x <- somethingThatReturnsIO somethingElseThatReturnsIO $ pureFunction x There is no way to get the “Int” out of an “IO Int”, except to do something else in the IO Monad. In monad terms, the above code desugars into somethingThatReturnsIO >>= (\x -> somethingElseThatReturnsIO $ pureFunction x) … Read more
As I said in a comment, the runtime objects allocated in memory don’t have type tags in a Haskell program. There is therefore no universal instanceof operation like in, say, Java. It’s also important to consider the implications of the following. In Haskell, to a first approximation (i.e., ignoring some fancy stuff that beginners shouldn’t … Read more