debugging - output a value in Haskell, with a default in case its type is not an instance of Show -

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this question has answer here:

i tried debug arrows , hard. end sticking trace here , there, wants show instance argument, limits uses. there way show if instance of show, , use kind of default output value if it's not? promise use debugging ;-)

how people debug arrows anyway...

your best bet compose trace arrow:

strace x = trace (show x) x  :: (num c, show c, arrow cat) => cat c c = arr (+1) <<< arr strace  -- > 1 -- 1 -- 2

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