Reverse Polish Notation (RPN) Calculator in Ruby and Haskell
A friend of mine is in DevBootCamp, an intensive coding school program designed to get students ready for real world job placement. Anyway, he was telling me how he was given an extra credit assignment: implement a RPN calculator which supports 3 functions — add, subtract, and multiply — on integer numbers. I.e., write a function
evaluate() that when given a string such as
"3 4 + 8 *" gives the result
56. He gave me his Ruby solution; in response, I wrote my own Ruby version and decided soon thereafter to write an equivalent Haskell version, because, why not?
First, the Ruby version:
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Pretty straightforward, yes? The
terms variable holds an array of numbers and operators. We use a calculator stack (aptly named,
stack) to untangle the RPN in a stack-based way.
each loop, the actions depend on what
term looks like. If
term is a number in string form, we convert it into a real number with
String#to_i, and push it into
term is one of the 3 recognized operators, we remove 2 terms
stack and apply the necessary operation on it with Ruby’s magic
Object#send method, and push the new term
c back into
stack for further operations (if any, in later iterations of the
while loop). Otherwise, we reject the input as an invalid term.
Here is the Haskell version:
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Probably the first thing to note is that we define a robust data type,
Term, to encapsulate the values held in a given RPN string. We take advantage of Haskell’s functions-as-first-class-values ability, and define the
TermOp constructor with it (i.e., it needs an arithmetic function
(Integer -> Integer -> Integer) as an argument).
The next thing to notice is that the
evaluate function is composed of smaller helper functions,
mkTerm simply converts a
String type into an appropriate
evalTerms takes a list of
Term values, and reduces it as much as possible by applying the
modifyStack function over it with
foldl (Haskell’s version of a single-pass loop).
modifyStack encounters a
TermInt, it pushes the number into the stack. When it encounters a
TermOp, it applies that operator to the first 2 items in
stack, and pushes this result back into
stack. We use pattern matching with
(a:b:_) to pull out the
b values from the stack — the expression
(a:b:_) means a value that matches either \([x_1, x_2]\), or \([x_1, x_2, ... , x_n]\), because the
_ operator matches anything, including the empty list
 used to finalize list creation. The
drop 2 stack is necessary because Haskell’s types by default are immutable.
I much prefer the Haskell version.
The separation of concerns is a big win — we can easily create helper functions like
evalTerms because of Haskell’s purity1. Haskell embraces the use of algebraic data types (i.e.,
Term here), and perhaps this preference lends itself to the use of helper functions that convert things from one type to another. Ruby does not have types, at least in the sense of Haskell types, so to artificially create such concepts and to implement them would be difficult.2 I mean, I really want to write equivalent
evalTerms methods in Ruby, but my beginner skills prevent me from doing it in a simple, straightforward way. I know enough about coding to abandon “solutions” that require circuitous, complex design.
I also like how all the functions are pure and thus easy to reason about with the type signatures. Refactoring code like this is a dream.
In short, algebraic data types, the clean delegation of subtasks to helper functions, and purity make the Haskell version easier to reason about and maintain in the long run.