I've been following a Learn Haskell in 10 Minutes tutorial. Everything was going well until I reached this line:
do { n <- readLn ; print (n^2) }
It won't execute in ghci, and in Leksah I get the following error.
Parse error: naked expression at top level
I've tried updating cabal and installing foo, but the problem continues.
Your help would be greatly appreciated.
I'm using Ubuntu 11.10.
It's not freezing, it's waiting for input. Try typing a number and pressing Enter.
Related
Vim has the possibility to let you replace selected text with the output of an external program. I'd like to take advantage of this with programs that I'd write in Haskell. But it doesn’t get the selected text as args.
-- show-input.hs
module Main where
import System.Environment
main = do
input <- getArgs
putStr ("Input was: " ++ (show input))
When I run it from the command line (NixOS GNU/Linux, BASH), I get the expected behavior:
$ ./show-input test
Input was: ["test"]
When I select some text in Vim and invoke :'<,'>!~/show-input, I get this :
Input was: []
There is something weird here, but I can't tell if it is from the way Vim passes arguments or from the way Haskell gets them. I have tried with both console Vim and graphical gVim (8.0.1451), with the same result.
NB: I can successfully use Vim Bang! with other external programs, such as grep. It works great.
---
Correct version after chepner's answer
So, for anyone interested, just replace getArgs with getContents and you get your input all in a string (instead of a list of strings).
module Main where
import System.Environment
main = do
input <- getContents
putStr ("Input was: " ++ (show input))
The ! command sends the seleted text to the program via standard input, not as a command line argument. The command line equivalent would be somecommand | ./show-input.
This question already has answers here:
Why doesn't Haskell sequence these IO actions properly?
(2 answers)
Closed 7 years ago.
I was playing around in Haskell and noticed something weird. I've defined a simple prompt function below.
-- file: test.hs
main :: IO ()
main = putStrLn . ("Hello, " ++) =<< (putStr "Name: " >> getLine)
When I runhaskell this, it works as expected, printing the prompt, waits for my input, then prints the greeting.
$ runhaskell test.hs
Name: kwarrtz
Hello, kwarrtz
When I compile it, however, things get weird. When I run it, it doesn't print the prompt, instead giving me a blank line and waiting for input. When I type my name and hit enter, it prints the prompt and the greeting, on the same line. In otherwords, the getLine happens before the putStr.
$ ghc test.hs
$ ./test
kwarrtz
Name: Hello, kwarrtz
Any thoughts on what's happening? I imagine it has something to do with the line buffering on my terminal, but I'm not sure how (that or I've just made some really ridiculous mistake in my code).
I'm running GHC 7.8.3 on Mac OS X El Capitan and using the default Terminal app.
Buffering.
hSetBuffering stdout NoBuffering
I think you can get all that from System.IO
when I tried to save the current ghci session using :save command I got the error
Prelude> :save
unknown command ':save'
use :? for help.
Prelude>
then when i type :? I got a ton of output that zoomed past the screen; how to read it one page at a time? is there anything like Unix less command there in ghci?
You could run GHCi from your shell and use shell redirection. This works for me on Windows:
echo :? | ghci > help.txt
more help.txt
I would expect this to also work on Unix, although I can't actually try it.
I have a bit of code in my haskell program like so:
evaluate :: String -> IO ()
evaluate = ...
repl = forever $ do
putStr "> " >> hFlush stdout
getLine >>= evaluate
Problem is, when I press the delete key (backspace on windows), instead of deleting a character from the buffer, I get a ^? character instead. What's the canonical way of getting delete to delete a character when reading from stdin? Similarly, I'd like to be able to get the arrow keys to move a cursor around, etc.
Compile the program and then run the compiled executable. This will give the correct behavior for the Delete key. For some reason interpreting the program screws up the use of Delete.
To compile the program, just invoke ghc like this:
$ ghc -O2 myProgram.hs
This will generate a myProgram executable that you can run from the command line:
$ ./myProgram
That will then give the correct behavior for Delete.
I've installed the Haskell platform on my mac (OSX lion), and ghci is running great.
Now I've created a haskell-file, stored on my "desk." How can I call it from this directory?
Example:
Prelude> :load datei.hs
[1 of 1] Compiling Main ( datei.hs, interpreted )
datei.hs:1:7: parse error on input `\'
Failed, modules loaded: none.
datei.hs:
let fac n = if n == 0 then 1 else n * fac (n-1)
Why do I get this?
Use the OSX terminal to reach your desktop and invoke yourfile.hs using ghci:
cd ~/Desktop
ghci yourfile.hs
edit:
As stated in the comments, the error message you're seeing above is warning you that the character \ exists at an unexpected location in the source code.
Since that character does not exist in the line of code you posted, there must be more to datei.hs. We need to see the rest of your source code before we can help.
If you saved your program with TextEdit, it's very possible that you're seeing a '\' character because you're saving it as an RTF file (TextEdit's default). Hit Ctrl-shift-t to convert it into a plain text file.
If your already in ghci you can use ':cd /path/to/file' as well.
Here is a good thread discussing let.