something rearranged in my brain when i saw some instructional haskell resource make the observation that fold is just replacing the constructors of a data type with a function that purrocesses them. it’s just the canonical destructor fur an inductively defined type
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(foldr f z '(1 2 3)) transfurms (cons 1 (cons 2 (cons 3 '()))) into (f 1 (f 2 (f 3 z)))
a list is a type with constructors
consandnil.foldtakes handlers fur both of those (nilbeing handled by just purroducing any arbitrary value). it recurses because the second argument toconsis a list