Likely like Elm, where indexing into an array returns a Maybe. OOB return Nothing, while in-bound returns Just <value>.

Or they don't support indexing at all.

Looks like they're using an indexed Algebraic Data Type.

https://github.com/grain-lang/grain/blob/master/src/grain-st...

There's no indexing in that.

Maybe by making the API of index based list datatypes return the value inside of a container type. In general it's a helpful strategy when creating total functions.