How to edit prior line in ghci's multi-line mode?

Question

Using ghci , I sometimes enter definitions using multi-line mode (as described in the interactive-evaluation documentation ).

For instance,

> :{
Prelude| sumToN :: (Eq a, Num a) => a -> a
Prelude| sumToN n
Prelude|   | signum n /= 1 = 0
Prelude|   | otherwise = 1 + sumToN (n-1)
Prelude| :}
>

When using this multi-line input mode, I occasionally realize that I have made a mistake in my input on a previous line. For example, in this code I accidentally typed A instead of a .

> :{
Prelude| sumToN :: (Eq a, Num a) => a -> A
Prelude|

To correct the mistake I need to edit a prior line. However, I cannot find any way to go back and edit a previous line. Instead, I ^C out of the multi-line mode and start again, repeating if necessary until I type the whole definition line-by-line without mistakes.

Is there any way to edit a previous line in ghci 's multi-line mode without having to type all lines again?

Answer 1

No. GHCi uses Haskeline , which only works on a per-line basis (and does not provide Cx Ce as GNU Bash does). Essentially, GHCi just works through a queue of lines , which use Haskeline's getInputLine .

So :{ and :} only take your code, add "\\n" inbetween your lines, and then "run it trough the interpreteter". There's no line-buffer you can edit, unfortunately. All you can do is to discard the lines with Cc and try again.