GHCi multiline prompt (`:set prompt2`) support

[konn]

GHCi has multiline input mode which can enabled by :set +m or :{ and :}.
For example:

ghci> :set +m
ghci> let a = 12
ghci|     b = 42
ghci| 
ghci>

:{ version:

ghci> :{
ghci> let a = 12
ghci|     b = 42
ghci| :}
ghci>

Prompt for multiline input (here, ghci|) can be set by :set prompt2 "ghci| ", but we don't have any configuration item to set prompt2 in haskell-mode.
This results in the following not-so-useful-for-indenting looking:

λ> let a = 12
ghci|     b = 42
ghci| 
λ> :{
λ> let a = 12
ghci|     b = 42
ghci| :}
λ> 

So we need the way to :set prompt2.

[gracjan]

Note that lambda is special and is detected by interactive-haskell-mode as command separator.

[issue-dispenser]

Doesn't interactive-haskell-mode use a defcustom for the prompt? What makes it special? I noticed that it's hardcoded into inferior-haskell-mode though.

What's stopping haskell-mode from using a new defcustom for prompt2?

[fice-t]

@konn You can now change it using the variable haskell-interactive-prompt2. The default is λ|.