I'm writing a small command-line utility in Haskell which should accept a command with an optional command-line argument - but if the argument is not present, the user should be prompted to enter it*. For example:
$ my_prog add item_name
Adding... done
$ my_prog add
Enter item name: item_name
Adding... done
My initial attempt looked something like this:
add args = do
let id = if length args > 0
then head args
else input where
input <- readLine
-- Do stuff with id
putStrLn id
Which fails to parse at the <-.
*I have since decided that this is a silly idea, but I thought I'd ask the question anyway.
You are attempting to use the do-notation inside the if, this will not work (and besides, won't typecheck since the whole if is outside the IO monad).
add args = do
id <- if length args > 0
then return $ head args
else readLine
putStrLn id
Related
Using cmdargs, is there a convenient way to print an error message and exit if a mandatory argument is missing? E.g. right now I have something like:
foo = Foo{script = def &= args &= typ "SCRIPT"}
main = do
scriptName <- script <$> cmdArgs foo
-- ...
If I run this program and don't pass the SCRIPT argument, scriptName is simply an empty string. Do I really have to manually check for and handle that using something like Control.Monad.Except?
I'm learning some Haskell and I came across this small program
reverseLines :: String -> String
reverseLines input =
unlines (map reverse (lines input))
main :: IO ()
main = interact reverseLines
This program will keep asking the user for more input and reverse the input and print it on the screen.
Most of this is straight forward but one thing I can't wrap my head around is why does this function keeps running and ask the user for more input whereas if I just replace the reverseLines function with a function the simply returns some string it will not happen.
This program will stop after one execution:
foo input = "Stops"
main :: IO ()
main = interact foo
Why?
If you look at the source of interact you see this:
interact f = do s <- getContents
putStr (f s)
see the getContents? This is where the magic starts - it will read everything till EOF
Now in Haskell this is lazy-IO which can be bad but here is almost magical - see the string is read lazily and passed to your reverseLines - this one of course will only generate output as soon as it saw \n characters (the lines) and so it seems your program is some kind of REPL.
In the second one you don't consume any of the lazy-string at all so it stops ASAP
As I wrote in the comments you can play with this by either passing content into the program using a file (or echo) and pipes on the terminal:
echo "Hello World\nBye Bye" | runhaskell LazyIO.hs
or using CTRL-D to pass in the EOF yourself.
To get a feeling for it I would play with the functions more - what happens if you use something that needs to see the complete input first (try reverse without the maps)? What happens with words instead of lines, ...?
Have fun!
I'm currently working my way through Learn You a Haskell for Great Good, and I'm trying to modify one of the code snippets in chapter nine, "Input and Output" to handle errors correctly:
main = do
(command:args) <- getArgs
let result = lookup command dispatch
if result == Nothing
then
errorExit
else
let (Just action) = result
action args
where
dispatch :: [(String, [String] -> IO ())]
is an association list
and
errorExit :: IO ()
is some function that prints an error message.
Compiling this with GHC gives the error message
todo.hs:20:13: parse error in let binding: missing required 'in'
which (to my understanding), seems to be saying that the "let" here doesn't realise it's in a "do" block.
Adding "do" on lines five and seven (after "then" and "else" respectively), changes the error message to
todo.hs:20:13:
The last statement in a 'do' block must be an expression
let (Just action) = result
todo.hs:21:5: Not in scope: `action'.
and now, whilst I agree with the first error message, I also have that one of my variables has jumped out of scope? I've double checked my alignment, and nothing seems to be out of place.
What is the appropriate way to assign a varaible within an if clause that is within a do block?
My suggestion is to not use if in the first place, use case. By using case you get to test the value and bind the result to a variable all in one go. Like this:
main = do
(command:args) <- getArgs
case lookup command dispatch of
Nothing -> errorExit
Just action -> action args
For a more in-depth discussion on why we should prefer case over if see boolean blindness.
#svenningsson suggested the right fix. The reason your original fails is because let clauses can only appear at the top level of a do block - they're simple syntactic sugar that doesn't look into inner expressions:
do let x = 1
y
desugars to the let expression
let x = 1 in y
Alas, in a do block, an expression clause like if ... then ... else ... has no way to declare variables in the rest of the do block at all.
There are at least two possible ways to get around this.
Absorb the remainder of the do block into the expression:
main = do
(command:args) <- getArgs
let result = lookup command dispatch
if result == Nothing
then
errorExit
else do
let (Just action) = result
action args
(This is essentially the method #svenningsson uses in his better case version too.)
This can however get a bit awkward if the remainder of the do expression needs to be duplicated into more than one branch.
("Secret" trick: GHC (unlike standard Haskell) doesn't actually require a final, inner do block to be indented more than the outer one, which can help if the amount of indentation starts getting annoying.)
Pull the variable declaration outside the expression:
main = do
(command:args) <- getArgs
let result = lookup command dispatch
action <- if result == Nothing
then
errorExit
else do
let (Just action') = result
return action'
action args
Here that requires making up a new variable name, since the pattern in the let clause isn't just a simple variable.
Finally, action was always out of scope in the last line of your code, but GHC works in several stages, and if it aborts in the parsing stage, it won't check for scope errors. (For some reason it does the The last statement in a 'do' block must be an expression check at a later stage than parsing.)
Addendum: After I understood what #Sibi meant, I see that result == Nothing isn't going to work, so you cannot use if ... then ... else ... with that even with the above workarounds.
You are getting an error because you are trying to compare values of function type. When you perform the check if result == Nothing, it tries to check the equality of Nothing with the value of result which is a type of Maybe ([String] -> IO ()).
So, if you want it to properly typecheck, you have to define Eq instances for -> and that wouldn't make any sense as you are trying to compare two functions for equality.
You can also use fmap to write your code:
main = do
(command:args) <- getArgs
let result = lookup command dispatch
print $ fmap (const args) result
I'm having problems with an exercise, and can not understand the error.
It should be a simple exercise with args:
import System.IO
import System.Environment
main= do
args < - getArgs
nomeficheiro <- return( args !! 0)
putStrnLn ( "Name is" ++ nomeficheiro)
Then i should run it, with : $ ./comando James
The error:
<interactive>:51:1:
parse error on input ‘$’
Perhaps you intended to use TemplateHaskell
I've read other doubts about args at this fórum and I didn't find any answer that could help me
$ ./comando James isn't meant to be run on GHCi. Instead, $ at the start of the line indicates that the following line should be run in your bash/cmd/shell, not in GHCi:
# in your favourite shell, in the correct directory
./comando James
If you want to run main with arguments within GHCi, you can use :main args:
ghci> :main James
Further remarks
Your current code isn't indented correctly, so make sure that you fix this too. Also, you can use let nomeficheiro = head args instead of … <- return …. Keep in mind that this could lead to problems if one doesn't supply any argument to your program, since head [] calls error.
I want to do a popen() / python's subprocess.communicate from Haskell - start a program, give it stdin, and get its stdout/stderr. What's the most direct / Haskellish way to do this?
Pick either MissingH's System.Cmd.Utils and the standard library's System.Process. They're easy to use, with both high-level convenience functions (which you simply throw strings at and get strings back, possibly lazily) and low-level plumbing functions (which actually give you handles, like most popen functions in other languages/frameworks).
import System.Process
main = do
let cmd = "mail"
args = ["root#localhost", "-s", "does this act like popen?"]
input = ["Hello, world!"]
(rc, out, err) <- readProcessWithExitCode cmd args input
putStrLn $ "mail exited: " ++ show rc
mapM_ putStrLn $ map ("out: " ++) $ lines out
mapM_ putStrLn $ map ("err: " ++) $ lines err
The other option could be to use shelly package