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-module(exp_parser). | |
-export([parse/1]). | |
% Parse simple math expressions. | |
% Currently works (for certain definitions of "works") | |
% for addition, subtraction and multiplication. | |
% Eventually it should handle a unary negation operator (~) | |
% The goal is to do the following: | |
% parse("(2 + 3)") => {plus, {num, 2}, {num, 3}} | |
% parse("((2 + 3) - 4)") => {minus, {plus, {num, 2}, {num, 3}, {num, 4}}} | |
% What actually happens is this: | |
% parse("(2 + 3)") => {expr, {plus, {num, 2}, {num, 3}}} | |
% parse("((2 + 3) - 4)") => {expr, {expr, {minus, {expr, {plus, {num, 2}, {num, 3}}}, {num, 4}}}} | |
% I can't seem to get rid of those darn outer "expr" tagged tuples | |
parse(Exp) -> | |
Tokens = tokenize(Exp), | |
{expr, Output} = inner_parse(Tokens), | |
Output. | |
%% The parsing functions | |
inner_parse([{TermL, L} | [{op, Op} | [{TermR, R} | Rest]]]) -> | |
inner_parse([ | |
{expr, {Op, {TermL, L}, {TermR, R} }} | | |
Rest | |
]); | |
inner_parse([{TermL, L} | [{op, Op} | [lparen | Rest]]]) -> | |
inner_parse([ | |
{expr, {Op, {TermL, L}, {expr, inner_parse(Rest) }}} | |
]); | |
inner_parse([lparen | Rest]) -> | |
{expr, inner_parse(Rest)}; | |
inner_parse([{expr, Exp} | [rparen | Rest]]) -> | |
inner_parse([{expr, Exp} | Rest]); | |
inner_parse([{expr, Exp}]) -> Exp. | |
%% "Tokenizing" functions (I'm not 100% sure "tokenize"" is what I mean here) | |
tokenize([]) -> []; | |
tokenize([32 | Rest]) -> tokenize(Rest); % ignore space characters | |
tokenize([$( | RestExp]) -> [lparen | tokenize(RestExp)]; | |
tokenize([$) | RestExp]) -> [rparen | tokenize(RestExp)]; | |
tokenize([Op | RestExp]) when Op =:= $+; Op =:= $-; Op =:= $* -> | |
[op(Op) | tokenize(RestExp)]; | |
tokenize([Digit | RestExp]) -> [digit(Digit) | tokenize(RestExp)]. | |
%% Tagging functions | |
op($+) -> {op, plus}; | |
op($-) -> {op, minus}; | |
op($*) -> {op, times}. | |
digit(Digit) -> {num, Digit - 48}. |
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