Functionally Solving Problems

This chapter of Learn You Some Erlang takes two problems borrowed from Learn You a Haskell and works through Erlang solutions for both, on the premise that once you understand functional concepts, the same shape of solution carries across languages with very different syntax.

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• The reverse Polish notation calculator tokenizes an expression, then folds over the tokens treating Erlang’s list as a stack: numbers get pushed, operators pop two values and push the result back.
• Reading a string as a number needs a small wrapper, since Erlang has no single built-in that parses both integers and floats.
• The Heathrow-to-London problem models the road as a list of {A,B,X} triples, one per stretch, with X the cost of crossing between the two roads.
• The accumulator for that fold is found by imagining standing in the middle of the algorithm: at each step, the shortest distance and path known so far for each side of the road.
• Both solutions favor lists:foldl over hand-rolled recursion once the recursive logic has been worked out on paper first.
• The chapter closes on breaking a problem into small solvable parts before coding it, and testing once it’s done, over any specific language trick.