In today's challenge, we're asked to interpret a series of basic command lines from a file and update the position of our submarine accordingly. It's basically a graph walk of sorts, with instructions of
forward. (Apparently you cannot go backward, as in life.)
As with yesterday's challenge, we could do it imperatively with a
foreach() loop and a couple of state variables floating around, but that conflates a whole bunch of different behaviors into one blob of code. We don't want to do that, so let's step back and consider the problem more clearly.
The first thing to realize is that we're tracking some value over time, our position, which is getting modified by each instruction line in the file. Said the other way around, we're iterating over a list of commands and each one is updating the position, using the result of the previous instruction as a starting point. "Walking a list and updating a value each time" should trigger our functional brains to say "ah ha, a reduce operation!"
As noted yesterday (I didn't know what today's challenge would be, I swear), a reduce operation walks over a list and performs the same operation (function) on each item, using the output of the previous iteration as an input. It also goes by the name "fold" or "foldl" (fold left) in a lot of more academic posts; same thing. But it fits our use case exactly: Given a start position and a list of "change position" commands, apply each command to the position in turn, returning a new position each time.
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