[Haskell-cafe] Arrow key functionality for REPL written in Haskell

[Mateusz Kowalczyk]

On 09/28/2014 09:43 AM, Travis Cardwell wrote:
> On 2014年09月28日 16:42, PENG, BO YA wrote:
>> I'm trying to implement the arrow key functionality for a systemf REPL to
>> recall previously entered lines. I think I should save the previous lines
>> in a list and then traverse through the list when an arrow key is pressed,
>> but I'm no sure how to detect an arrow key and implement this functionality.
>> Can anyone give a brief description about the implementation or point me to
>> some other resources?
>> Thanks!
> 
> Perhaps readline would work for you?
> 
> http://hackage.haskell.org/package/readline
> 
> Cheers,
> 
> Travis
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For keeping track itself, you can use something like a pointed list[1]
pretty easily which frees you from having to keep track of indexes,
looping around, going back and forth &c. I use it for quick-and-dirty
selection in a menu in the game I'm writing. Once you have a way to get
key presses, it becomes trivial (+ some lenses for clarity):

whenM (keyPress KeyUp) $ screenState . maps %= C.previous
whenM (keyPress KeyDown) $ screenState . maps %= C.next

where ‘maps’ is my PointedList and C is Data.List.Pointed.Circular.

[1]: http://hackage.haskell.org/package/pointedlist

-- 
Mateusz K.