Error messages

The compiler does its best to tell us when something seems amiss in our code. Let’s intentionally introduce a “mistake” and see what happens.

Remove the closing quotation mark in the hello world program, and save the file.

main = putStrLn "hello world

The display in ghcid now displays an error message.

hello-world.hs:1:29: error:
    lexical error in string/character literal at character '\n'

The output includes an excerpt from the source file with a marker ^ pointing to the location of the error. This is a particularly useful way to see at a glance where the problem is.

1 | main = putStrLn "hello world
  |                             ^