How to append to List?

[refaelsh]

Hi,

There is no append function to List.

1. Why? I was surprised there is no such a function. Seems natural there should be such a function.
2. How can I append to List? Please help :-)

[jtdaugherty]

1. Because a List is not intended to be used as a mutable data structure with all of the usual "list" operations; it is a UI representation of a list, and the state changes in the list should be deliberate. But this shouldn't be a problem, because ...
2. Your best bet is likely to use listReplace to provide the new element sequence you want to store in the list. If you want to get the elements from the list and then modify that sequence in some way (such as to put it back with listReplace), you can use listElements.

[refaelsh]

Because a List is not intended to be used as a mutable data structure with all of the usual "list" operations; it is a UI representation of a list, and the state changes in the list should be deliberate. But this shouldn't be a problem, because ...

I understand.

Your best bet is likely to use listReplace to provide the new element sequence you want to store in the list. If you want to get the elements from the list and then modify that sequence in some way (such as to put it back with listReplace), you can use listElements.

I dont get it. How listReplace helps me to append? Can you please provide a small example?

[jtdaugherty]

The function to modify the list would be something like \lst -> listReplace (listElements lst <> elementsToAppend) (Just i) lst where i is the index of the element that you want selected after the modification. In the context of EventM, you'd either use this with a lens (i.e. assuming appStateList is a lens made from the _appStateLens field of your application state type) like

lst <- use appStateList
let newElements = listElements lst <> elementsToAppend
appStateList %= listReplace newElements (Just i)

or without a lens (i.e. assuming appStateList is just a record field of your application state type) like

lst <- gets appStateList
let newElements = listElements lst <> elementsToAppend
modify $ \s -> s { appStateList = listReplace newElements (Just i) lst }

[refaelsh]

I understand. Thank you very much.

[jtdaugherty]

Okay, great, I'm glad that is helpful!