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.

Reading notes

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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.