reduce returns first value in array when accumulator is null

Array.reduce as of Godot 4.4 will use the first element in the array when the accumulator is either undefined or explicitly set to null by the user, contrary to how JavaScript, F#, and Python works. ¹

This means that you cannot write a function to pass to reduce that would fold over a list and return the best suitable match based on some criteria or null if none of those elements are appropriate. This is because it could keep picking an inappropriate result if one exists at the front of the list. ¹

If you are familiar with Haskell, another way of thinking about it is that reduce in GDScript silently turns into foldl1 when Nothing is passed to foldl as the initial value for the accumulator. ¹

This behavior is documented in the documentation for reduce, this is because of how the accum parameter works: ¹

The method takes two arguments: the current value of accum and the current array element. If accum is null (as by default), the iteration will start from the second element, with the first one used as initial value of accum.

Footnotes

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