Is there a way to chain :reload/:r along with :main as a single command in GHCi?
The goal here is to avoid typing both every time I change something in my other terminal, but to just type ↑Enter.
:cmd seems to accept string with multiple lines.
Therefore you can do the following command.
:cmd return $ unlines [":reload",":main"]
also you can add following code to ~/.ghci
:def hoge const $ return $ unlines [":reload",":main"]
now you can execute :hoge in ghci
Related
I'm new to Haskell and I have been using system to execute shell commands for me, since I'm only interested in exit status. I noticed that since I can do something like:
ls <- system "ls"
pwd <- system "pwd"
and this correctly executes the two commands. I was thinking about executing an array of these commands eg
lsAndPwd <- return $ system <$> ["ls", "pwd"]
and I'm surprised that this doesn't actually execute anything. This compiles and I checked that lsAndPwd has the correct type of [IO GHC.IO.Exception.ExitCode] but the commands are never executed. What's going on and how can I get this to work?
You should use mapM instead of <$> so you get an IO [ExitCode], not an [IO ExitCode]. This way, it's collected into one IO monad that you can run:
lsAndPwd <- mapM system ["ls", "pwd"]
Alternatively, call sequence on the whole thing:
lsAndPwd <- sequence $ system <$> ["ls", "pwd"]
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.
Intuitively, I've tried
$ ghc -e "import System.Environment" -e "getArgs" -- a b c
ghc: unrecognised flag: --
did you mean one of:
-n
-F
-v
Usage: For basic information, try the `--help' option.
$
...without success.
I was expecting the output to be along the lines of ["a","b","c"].
The docs here don't seem to mention any way to pass cli arguments through.
AFAIK piping in stdio works as expected.
And perhaps if it is not possible to pass in arguments with ghc -e, maybe it's still possible with ghci to supply both some code, some arguments to it, run, then exit?
To answer the second part of the question:
Prelude> :help :main
Commands available from the prompt:
…
:main [<arguments> ...] run the main function with the given arguments
…
Prelude> let main = System.Environment.getArgs >>= print
Prelude> :main foo bar
["foo","bar"]
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
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.