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The error was my own. Only I couldn't figure out where exactly.

Such errors can't be avoided. I need arrays for performance.

If performance is no concern I can use Maps, but I still need indexing ("partial functions") for architectural reasons: separation of concerns - can't make "safe" graphs where everything holds a reference to everything "related".

I just don't buy the idea that partial functions can be avoided.

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I.e. look at how PureScript does it [0]. Haskell could do the same.

[0] https://pursuit.purescript.org/packages/purescript-arrays/3....

Sure, I can use a Maybe variant, but the consequence would be that I wrap the whole program in a layer of maybes for cases that I "know" shouldn't happen but happened -- because I do make mistakes.

Or in other words, an "AssertionMonad". No thanks, I'm perfectly fine with assertion errors (i.e. "Index out of bounds"). I just need help with finding my own errors. Stacktraces are a good thing.

I agree, stack traces are a good thing.

For the time being, make a helper function:

ixWithString :: String -> Int -> Array a -> a

check the bounds inside and error out with a custom message when the index is outside the array bounds.

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