[nostrademons]

A lexer is a fold as well - there are usually fewer elements in the output list than in the input list. A mapping function implies a 1:1 correspondence, because the recursion is abstracted out into map() and the map function can't accumulate any state during the computation. (At least in traditional FP technology; the Mapper in MapReduce isn't actually a true map function, it's more like an unfold.)

[ch]

Ah true I am incorrect on the lexer as a map. Had I thought on that half a second more I would have saved myself some embarrassment. :)