How to get a tuple's field using a runtime index in a type stable way?

General Usage
5

As you can tell, it took a few tries :smiley:

function test4(tup,idx,typ::Type{T}) where T
       idx == 1 && (x=first(tup); isa(x,T) && return x)
       test4(Base.tail(tup),idx-1,T)
end

This seems to do the trick and not allocate (throws an error otherwise), it is also type stable. outer_test would not allocate regardless of inlining.

Hope this helps…

Edit:
This seems to do the same and simpler:

function test4(tup,idx,typ::Type{T}) where T
       idx == 1 && (x=first(tup); return x::T) 
       #or if you want an explicit error
       #isa(x,T) ? (return x) : error("Bla"))
       test4(Base.tail(tup),idx-1,T)
end
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