ghci fails to set breakpoint

Question

I have an extremely simple Main.hs that works fine as a compiled program:

module Main( main ) where
import System.Environment ( getArgs )

atoi :: String -> Int
atoi s = read s :: Int

main :: IO ()
main = do

(x:y:_) <- getArgs

case compare (atoi x) (atoi y) of
  LT -> putStrLn "x < y"
  GT -> putStrLn "x > y"
  EQ -> putStrLn "x = y"

When I try to debug it with:

$ ghci Main.hs
:set args 55 26667
break main

I get the following error:

<interactive>:2:7: error:
    • Couldn't match expected type ‘a -> Bool’ with actual type ‘IO ()’
    • In the first argument of ‘break’, namely ‘main’
      In the expression: break main
      In an equation for ‘it’: it = break main
    • Relevant bindings include
        it :: [a] -> ([a], [a]) (bound at <interactive>:2:1)
*Main> 

What am I doing wrong?

Answer 1

GHCi commands start with a : to distinguish them from Haskell expressions. break is the name of a Haskell function to split a list on a given condition. To use GHCi breakpoints, use :break instead.