How can I make a "copy list (A) into list (B)" custom block?

[towerofnix]

Suppose I want to create a custom block which takes all the items in a given list and copies them into another given list. Hypothetically speaking, this is a quite simple - but useful - custom block:

However, it is impossible to drag blocks into list dropdowns, and therefore impossible to create a general custom block like this.

In a very real sense, no general custom blocks which apply to lists can be created. This is a barrier for teaching the uses of custom blocks and a hindrance to anybody who would otherwise make use of such custom blocks.

See also scratchfoundation/scratch-parser#47, which may be required or useful in allowing blocks to be placed into list dropdowns. Also see #1805, which recommends adding a "sprite dropdown" input type - in the same way, adding a "list dropdown" input type would be useful for general custom blocks like this.

[apple502j]

See also scratchfoundation/scratch-vm#1030

[thisandagain]

/cc @carljbowman @kchadha @ericrosenbaum

[Wowfunhappy]

See also: scratchfoundation/scratch-gui#2194