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local lpeg = require"lpeglabel"
local R, S, P, V, C, Ct, T = lpeg.R, lpeg.S, lpeg.P, lpeg.V, lpeg.C, lpeg.Ct, lpeg.T
local labels = {
{"NoExp", "no expression found"},
{"Extra", "extra characters found after the expression"},
{"ExpTerm", "expected a term after the operator"},
{"ExpExp", "expected an expression after the parenthesis"},
{"MisClose", "missing a closing ')' after the expression"},
}
local function expect(patt, labname)
for i, elem in ipairs(labels) do
if elem[1] == labname then
return patt + T(i)
end
end
error("could not find label: " .. labname)
end
local num = R("09")^1 / tonumber
local op = S("+-*/")
local function compute(tokens)
local result = tokens[1]
for i = 2, #tokens, 2 do
if tokens[i] == '+' then
result = result + tokens[i+1]
elseif tokens[i] == '-' then
result = result - tokens[i+1]
elseif tokens[i] == '*' then
result = result * tokens[i+1]
elseif tokens[i] == '/' then
result = result / tokens[i+1]
else
error('unknown operation: ' .. tokens[i])
end
end
return result
end
local g = P {
"Exp",
Exp = Ct(V"Term" * (C(op) * expect(V"Term", "ExpTerm"))^0) / compute;
Term = num + V"Group";
Group = "(" * expect(V"Exp", "ExpExp") * expect(")", "MisClose");
}
g = expect(g, "NoExp") * expect(-P(1), "Extra")
local function eval(input)
local result, label, suffix = g:match(input)
if result ~= nil then
return result
else
local pos = input:len() - suffix:len() + 1
local msg = labels[label][2]
return nil, "syntax error: " .. msg .. " (at index " .. pos .. ")"
end
end
print(eval "98-76*(54/32)")
--> 37.125
print(eval "(1+1-1*2/2")
--> syntax error: missing a closing ')' after the expression (at index 11)
print(eval "(1+)-1*(2/2)")
--> syntax error: expected a term after the operator (at index 4)
print(eval "(1+1)-1*(/2)")
--> syntax error: expected an expression after the parenthesis (at index 10)
print(eval "1+(1-(1*2))/2x")
--> syntax error: extra chracters found after the expression (at index 14)
print(eval "-1+(1-(1*2))/2")
--> syntax error: no expression found (at index 1)
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