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-- Matthew's recovery example
lpeg = require'lpeglabel'
local R, S, P, V = lpeg.R, lpeg.S, lpeg.P, lpeg.V
local C, Cc, Ct, Cmt = lpeg.C, lpeg.Cc, lpeg.Ct, lpeg.Cmt
local T = 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 errors = {}
local function expect(patt, labname)
function recorderror(input, pos)
table.insert(errors, {labname, pos})
return true
end
return patt + Cmt("", recorderror) * T(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) * V"Operand")^0) / compute;
Operand = expect(V"Term", "ExpTerm");
Term = num + V"Group";
Group = "(" * V"InnerExp" * expect(")", "MisClose");
InnerExp = expect(V"Exp", "ExpExp");
ExpTerm = Cc(0);
MisClose = P"";
ExpExp = (P(1) - ")")^0 * Cc(0);
}
g = expect(g, "NoExp") * expect(-P(1), "Extra")
local function eval(input)
local result, label, suffix = g:match(input)
if #errors == 0 then
return result
else
local out = {}
for i, err in ipairs(errors) do
local pos = err[2]
local msg = labels[err[1]]
table.insert(out, "syntax error: " .. msg .. " (at index " .. pos .. ")")
end
errors = {}
return nil, table.concat(out, "\n")
end
end
assert(eval "98-76*(54/32)" == 37.125)
--> 37.125
local e, msg = eval "(1+1-1*2/2"
assert(e == nil and msg == "syntax error: missing a closing ')' after the expression (at index 11)")
e, msg = eval "(1+)-1*(2/2)"
assert(e == nil and msg == "syntax error: expected a term after the operator (at index 4)")
e, msg = eval "(1+1)-1*(/2)"
assert(e == nil and msg == "syntax error: expected an expression after the parenthesis (at index 10)")
e, msg = eval "1+(1-(1*2))/2x"
assert(e == nil and msg == "syntax error: extra characters found after the expression (at index 14)")
e, msg = eval "-1+(1-(1*2))/2"
assert(e == nil and msg == "syntax error: no expression found (at index 1)")
e, msg = eval "(1+1-1*(2/2+)-():"
assert(e == nil and msg == "syntax error: expected a term after the operator (at index 13)\n" ..
"syntax error: expected an expression after the parenthesis (at index 16)\n" ..
"syntax error: missing a closing ')' after the expression (at index 17)\n" ..
"syntax error: extra characters found after the expression (at index 17)")
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