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# randrews/hoc4.lua

Created Jun 21, 2015
Toy calculator in Lua, version 4
 setmetatable(_ENV, { __index=lpeg }) Scopes = { {} } function eval_expr(expr) local accum = eval(expr[2]) -- because 1 is "expr" for i = 3, #expr, 2 do local operator = expr[i] local num2 = eval(expr[i+1]) if operator == '+' then accum = accum + num2 elseif operator == '-' then accum = accum - num2 elseif operator == '*' then accum = accum * num2 elseif operator == '/' then accum = accum / num2 end end return accum end function eval_bool(expr) local num1 = eval(expr[2]) local operator = expr[3] local num2 = eval(expr[4]) if operator == '<' then return num1 < num2 elseif operator == '<=' then return num1 <= num2 elseif operator == '>' then return num1 > num2 elseif operator == '>=' then return num1 >= num2 elseif operator == '==' then return num1 == num2 elseif operator == '!=' then return num1 ~= num2 end end function eval(ast) if type(ast) == 'number' then return ast elseif ast[1] == 'expr' or ast[1] == 'term' then return eval_expr(ast) elseif ast[1] == 'array' then local new = {} for _, el in ipairs(ast[2]) do table.insert(new, eval(el)) end return new elseif ast[1] == 'ref' then return lookup(ast) elseif ast[1] == 'assign' then return assign(ast[2], eval(ast[3])) elseif ast[1] == 'list' then local last = nil for i = 2, #ast do last = eval(ast[i]) end return last elseif ast[1] == 'if' then if eval_bool(ast[2]) then return eval(ast[3]) end elseif ast[1] == 'while' then while eval_bool(ast[2]) do eval(ast[3]) end elseif ast[1] == 'function' then return { args=ast[2], code=ast[3], scope=Scopes[#Scopes] } elseif ast[1] == 'call' then local fn = eval(ast[2]) local scope = setmetatable({}, {__index=fn.scope}) for i, name in ipairs(fn.args) do scope[name] = eval(ast[3][i]) end table.insert(Scopes, scope) local result = eval(fn.code) table.remove(Scopes) return result end end function assign(ref, value) local current = Scopes[#Scopes] for i = 2, #ref do local next_index = ref[i] if type(next_index) == 'table' then next_index = eval(next_index) end if i == #ref then -- last one, set the value -- Special case, assign to something in an outer scope while current[next_index] and not rawget(current, next_index) do -- Walk the scope chain back until we see it current = getmetatable(current).__index end current[next_index] = value return value else -- not the last, keep following the chain current = current[next_index] end end end function lookup(ref) local current = Scopes[#Scopes] for i = 2, #ref do local next_index = ref[i] if type(next_index) == 'table' then next_index = eval(next_index) end current = current[next_index] end return current end spc = S(" \t\n")^0 digit = R('09') number = C( (P("-") + digit) * digit^0 * ( P('.') * digit^0 )^-1 ) / tonumber * spc lparen = "(" * spc rparen = ")" * spc lbrack = "[" * spc rbrack = "]" * spc lcurly = "{" * spc rcurly = "}" * spc comma = "," * spc expr_op = C( S('+-') ) * spc term_op = C( S('*/') ) * spc letter = R('AZ','az') name = C( letter * (digit+letter+"_")^0 ) * spc keywords = (P("if") + P("while") + P("function")) * spc name = name - keywords boolean = C( S("<>") + "<=" + ">=" + "!=" + "==" ) * spc stmt = spc * P{ "LIST"; LIST = V("STMT") + Ct( Cc("list") * lcurly * (V("STMT") * P(";")^0 * spc)^0 * rcurly ), STMT = Ct( Cc("assign") * V("REF") * "=" * spc * V("VAL") ) + V("IF") + V("WHILE") + V("CALL") + V("FUNCTION") + V("EXPR"), EXPR = Ct( Cc("expr") * V("TERM") * ( expr_op * V("TERM") )^0 ), TERM = Ct( Cc("term") * V("FACT") * ( term_op * V("FACT") )^0 ), REF = Ct( Cc("ref") * name * (lbrack * V("EXPR") * rbrack)^0 ), FACT = number + lparen * V("EXPR") * rparen + V("REF"), ARRAY = Ct( Cc("array") * lbrack * V("VAL_LIST") * rbrack ), VAL_LIST = Ct( (V("VAL") * comma^-1)^0 ), VAL = V("CALL") + V("ARRAY") + V("FUNCTION") + V("EXPR"), BOOL = Ct( Cc("bool") * V("EXPR") * boolean * V("EXPR") ), IF = Ct( C("if") * spc * lparen * V("BOOL") * rparen * V("LIST") ), WHILE = Ct( C("while") * spc * lparen * V("BOOL") * rparen * V("LIST") ), CALL = Ct( Cc("call") * V("REF") * spc * lparen * V("VAL_LIST") * rparen ), FUNCTION = Ct( C("function") * spc * lparen * V("NAME_LIST") * rparen * V("LIST") ), NAME_LIST = Ct( (name * comma^0)^0 ) } function test(stmt) local Global = Scopes[1] stmt = stmt / eval assert(stmt:match(" 1 + 2 ") == 3) assert(stmt:match("1+2+3+4+5") == 15) assert(stmt:match("2*3*4 + 5*6*7") == 234) assert(stmt:match(" 1 * 2 + 3") == 5) assert(stmt:match("( 2 +2) *6") == 24) stmt:match("a=3"); assert(Global.a == 3) assert(stmt:match("a") == 3) assert(stmt:match("a * 5") == 15); Global.a=nil stmt:match("a = [ 4, 5, 6 ]"); assert(Global.a[1] == 4) assert(Global.a[2] == 5) assert(Global.a[3] == 6) Global.a=nil stmt:match("b = [ ]"); assert(Global.b[1] == nil) Global.b=nil stmt:match("c = [[1,2], [3,4]]") assert(Global.c[1][1] == 1) assert(Global.c[1][2] == 2) assert(Global.c[2][1] == 3) assert(Global.c[2][2] == 4) assert(stmt:match("c[4/2][1]") == 3) stmt:match("c[3] = 5") assert(Global.c[3] == 5) Global.c=nil stmt:match("if(1 < 0) b = 5"); assert(Global.b ~= 5) Global.n=0; Global.x=1 stmt:match("while(n < 8) { x = x * 2; n = n + 1 }") assert(Global.x == 256) Global.n=nil; Global.x=nil stmt:match("f = function(a) a*2+3") assert(Global.f ~= nil) assert(Global.f.scope == Scopes[1]) stmt:match("res = f(5)") assert(Global.res == 13) assert(Global.a == nil) assert(#Scopes == 1) Scopes[1] = {}; Global = Scopes[1] stmt:match("make_acc = function(){ a = 0; function(n) a = a+n; }") assert(Global.make_acc) stmt:match("acc = make_acc()") assert(stmt:match("acc(1)") == 1) assert(stmt:match("acc(1)") == 2) end function repl(file) file = file or io.input() parser = stmt for line in file:lines() do print(parser:match(line)) end end

### sourcevault commented Sep 13, 2018 • edited

 @randrews Do you know why somebody would use `lpeg.Cmt` ? In moonscript it seems to be used : leafo/moonscript@2f09192#diff-272b1f163aec9b0bd2b9b5dab4a9688dR117 How is it different than normal capture ? In your complete programming language where would somebody use it ? Thanks !

### sourcevault commented Apr 4, 2019

 To anybody reading `lpeg.Cmt` is a better `/f` way to call function. If you are seriously creating a programming language then use `lpeg.Cmt` instead of `/` for attaching functions to your parser.
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