getting <<loop>> message when run haskell programme

Question

I have written code like this

import System.Environment
import Control.Exception
import Data.List

f :: String -> [String] -> IO ()
f str []       =
    putStrLn "String 2"
f str (x : xs) =
    putStrLn "String 1"

main :: IO ()
main = do
    xs <- getArgs
    let str = head xs
    let xs = tail xs
    f str xs
    return ()

but when I compile it and trying to run I always have only answer <<loop>>

ghc run.hs
./run some_string some_over_arguments
run: <<loop>>

What's wrong with this code? I've tryed to hoogle <<loop>> but found nothing. If I pass to f not str xs but str [some_hardcoded_list] this code works fine, so I guess this is something wrong with xs.

So what input args are you using? This is what gets assigned to xs. Please paste the whole run command. — EkcenierK
– EkcenierK, Commented Nov 14, 2015 at 10:37
@KLibby for example $ ./run my_string any_argument this is exactly what I tiped — PepeHands
– PepeHands, Commented Nov 14, 2015 at 10:39
Possible duplicate of infinite loop in Haskell when binding variables — Zeta
– Zeta, Commented Nov 14, 2015 at 10:57
You should compile with -Wall; when you do, the compiler will warn you when these things happen. And just a by the way: instead of binding the head and then the tail you can just pattern match the _:_ constructor. It's easier to write and if you do, the compiler will also warn you about the second potential problem in your code (inexhaustive pattern match -- what if xs is []?). And besides that incomplete functions like head & tail are just bad :P — MasterMastic
– MasterMastic, Commented Nov 14, 2015 at 11:08
@MasterMastic thanks, I did not know that -Wall exists in haskell. — PepeHands
– PepeHands, Commented Nov 14, 2015 at 11:17

Random Dev · Accepted Answer · 2015-11-14 10:42:48Z

6

to you see the part where you declare

let xs = tail xs

this says you want xs to be a list that is the tail of itself and this causes an infinte loop (which ghc seems to notice ;)) (seen as an definition every constant list repeat c would suffice here - technically of course you just get a plain infinite-loop)

so just change it to something like

main :: IO ()
main = do
    xs <- getArgs
    let str = head xs
    let xs' = tail xs
    f str xs'
    return ()

and your code should work as expected ;)

edited Nov 14, 2015 at 10:42

answered Nov 14, 2015 at 10:40

Random Dev

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chi · Accepted Answer · 2015-11-14 11:05:28Z

3

Others have already answered. I just want to add that enabling warnings will point out redefinitions of existing bindings. This also covers this kind of unwanted recursion:

> :set -Wall
> xs <- return [1..10::Int]
> let xs = tail xs

<interactive>:11:5: Warning:
    This binding for ‘xs’ shadows the existing binding
      defined at <interactive>:10:1

Now, one could think "but in this case I really intend to shadow the previous binding", and would be completely correct. Still, renaming the new binding incidentally avoids the problem.

One more reason to add {-# OPTIONS -Wall #-} at the top of your source file. (Or enabling warning in your .cabal file, or whatever other build system)

answered Nov 14, 2015 at 11:05

chi

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