Thursday, April 14, 2011

A little gotcha in overloading comparison methods in Python

Python supports chaining relational operators so you can express 2 < 3 < 4 and get true. It looks like this is implemented as a little compiler trick that actually creates the expression "2 < 3 and 3 < 4". This can be confusing if you try to do something insane like implement Haskell's (>>=) in Python using (>=). Something like:
Just(10) >= (lambda x : Just(x + 1)) >= (lambda x : Just(x / 2))
Will actually become:
Just(10) >= (lambda x : Just(x + 1)) and \
    (lambda x : Just(x + 1)) >= (lambda x : Just(x / 2))
This only seems to only apply to the relational operators, choosing to implement (>>=) with (>>) in Python seems to work fine. Here is a Gist showing the problem from @apgwoz: https://gist.github.com/916132.

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