Reverse Polish Notation Calculator in Erlang

calc.erl

%% @doc Reverse Polish Notation Calculator.
%%
%% Parses expressions like "1 2 3 + -" = -4
%%
%% This is an exercise in Learn You some Erlang for Great Good,
%% however I didn't read the text and just implemented it.
%%
%% I guess understanding stack-based parsing helps here.
-module(calc).
-export([rpn/1]).

%% @doc Parse and calculate the RPN expression from a string.
%%
%% @spec rpn("1 2 3 + -") -> -4
%% @spec rpn("1 2 3 -") -> throw(invalid_expr)
rpn(Expr) ->
  parse(string:tokens(Expr, " "), []).

%% @private
parse([], Stack) ->
  case length(Stack) of
    1 -> hd(Stack);
    _ -> throw(invalid_expr)
  end;
parse([Token|Tail], Stack) ->
  case Token of
    "+" ->
      parse(Tail, reduce(fun(A, B) -> A + B end, Stack));
    "-" ->
      parse(Tail, reduce(fun(A, B) -> A - B end, Stack));
    "*" ->
      parse(Tail, reduce(fun(A, B) -> A * B end, Stack));
    "/" ->
      parse(Tail, reduce(fun(A, B) -> A / B end, Stack));
    _ ->
      parse(Tail, [list_to_integer(Token)|Stack])
  end.

%% @private
reduce(F, Stack = [A,B|Tail]) ->
  case length(Stack) < 2 of
    true  -> throw(invalid_expr);
    false -> [F(B, A)|Tail]
  end.

calc2.erl

%% @doc Reverse Polish Notation Calculator.
%%
%% Parses expressions like "1 2 3 + -" = -4
%%
%% This is an exercise in Learn You some Erlang for Great Good
%% and this is the succinct solution in the book.
-module(calc).
-export([rpn/1]).

%% @doc Parse and calculate the RPN expression from a string.
%%
%% @spec rpn("1 2 3 + -") -> -4
rpn(Expr) ->
  hd([_] = lists:foldl(fun parse/2, [], string:tokens(Expr, " "))).

%% @private
parse("+", [A,B|Tail]) ->
  [B+A|Tail];
parse("-", [A,B|Tail]) ->
  [B-A|Tail];
parse("*", [A,B|Tail]) ->
  [B*A|Tail];
parse("/", [A,B|Tail]) ->
  [B/A|Tail];
parse(Token, Stack) ->
  [list_to_integer(Token)|Stack].