LPegLabel

## LPegLabel - Parsing Expression Grammars (with Labels) for Lua --- ### Introduction LPegLabel is a conservative extension of the [LPeg](http://www.inf.puc-rio.br/~roberto/lpeg) library that provides an implementation of Parsing Expression Grammars (PEGs) with labeled failures. Labels can be used to signal different kinds of errors and to specify which recovery pattern should handle a given label. Labels can also be combined with the standard patterns of LPeg. Besides that, LPegLabel also reports the farthest failure position in case of an ordinary failure (which is represented by label **fail**). This document describes the new functions available in LpegLabel and presents some examples of usage. With labeled failures it is possible to distinguish between an ordinary failure and an error. Usually, an ordinary failure is produced when the matching of a character fails, and this failure is caught by ordered choice. An error (a non-ordinary failure), by its turn, is produced by the throw operator and may be caught by a recovery rule. In LPegLabel, the result of an unsuccessful matching is a triple **nil, lab, errpos**, where **lab** is the label associated with the failure (a string or an integer), and **errpos** is the input position being matched when **lab** was thrown. When **lab** is an ordinary failure and no error was thrown before, **errpos** is the farthest position where an ordinary failure occurred. In case **lab** is an ordinary failure and an error was thrown before, **errpos** is the farthest input position where an ordinary failure occurred after the last error. Below there is a brief summary of the new functions provided by LpegLabel:
FunctionDescription
lpeglabel.T (l) Throws a label l to signal an error
%{l} Syntax of relabel module. Equivalent to lpeglabel.T(l)
p^l Syntax sugar available at relabel for p / %{l}
relabel.calcline(subject, i) Calculates line and column information regarding position i of the subject
### Functions #### lpeglabel.T(l) Returns a pattern that throws the label `l`, which can be an integer or a string. When a label is thrown, the current subject position is used to set **errpos**, no matter whether it is the fartherst failure position or not. In case the PEG grammar has a rule `l`, after a label is thrown this rule will be used as a recovery rule, otherwise the whole matching fails. The recovery rule will try to match the input from the subject position where `l` was thrown. In case the matching of the recovery rule succeeds, the regular matching is resumed. Otherwise, the result of the recovery rule is the matching result. When we have a predicate such as `-p` or `#p` and a label `l` is thrown during the matching of `p`, this causes the failure of `p`, but does not propagate `l`, or calls its associated recovery rule. #### %{l} Syntax of *relabel* module. Equivalent to `lpeglabel.T(l)`. Label `l` must be a valid identifier name. #### p^l Syntax of *relabel* module. The pattern `p^l` is equivalent to `p + lpeglabel.T(l)`. Label `l` must be a valid identifier name. #### relabel.calcline (subject, i) Returns line and column information regarding position i of the subject. ### Examples Below there a few examples of usage of LPegLabel. The code of these and of other examples is available in the *examples* directory. #### Reporting the farthest failure This example illustrates the new values returned by the *match* function in case of an unsuccessful matching. As no error is thrown in this example, when the matching fails *errpos* represents the farthest suffix where an ordinary failure occurred. ```lua local m = require'lpeglabel' function matchPrint(p, s) local r, lab, errpos = p:match(s) print("r: ", r, "lab: ", lab, "errpos: ", errpos) end local p = m.P"a"^0 * m.P"b" + m.P"c" matchPrint(p, "abc") --> r: 3 lab: nil errpos: nil matchPrint(p, "c") --> r: 2 lab: nil errpos: nil matchPrint(p, "aac") --> r: nil lab: fail errpos: 3 matchPrint(p, "xxc") --> r: nil lab: fail errpos: 1 ``` #### Matching a list of identifiers separated by commas The following example defines a grammar that matches a (possibly empty) list of identifiers separated by commas. A label is thrown when there is no identifier after a comma, or when the whole input is not matched. ```lua local m = require'lpeglabel' local re = require'relabel' local terror = { ErrId = "expecting an identifier", ErrEnd = "expecting EOF", fail = "undefined" } local id = m.R'az'^1 local g = m.P{ 'S', S = m.V'List' * (-m.P(1) + m.T'ErrEnd'), List = m.V'Id' * (m.V'Comma' * (m.V'Id' + m.T'ErrId'))^0, Id = m.V'Sp' * id, Comma = m.V'Sp' * ',', Sp = m.S' \n\t'^0, } function mymatch (g, s) local r, e, pos = g:match(s) if not r then local line, col = re.calcline(s, pos) local msg = "Error at line " .. line .. " (col " .. col .. "): " return r, msg .. terror[e] .. " before '" .. s:sub(pos) .. "'" end return r end print(mymatch(g, "one,two")) --> 8 print(mymatch(g, "one two")) --> nil Error at line 1 (col 4): expecting EOF before ' two' print(mymatch(g, "one,\n two,\nthree,4")) --> nil Error at line 3 (col 7): expecting an identifier before '4' ``` In this example we could think about writing rule List as follows: ```lua List = m.V'Id' * ((m.V'Comma' + m.T'ErrComma') * (m.V'Id' + m.T'ErrId'))^0, ``` but when matching m.V'Comma' + m.T'ErrComma' against the end of input we would get a failure whose associated label would be **errComma**, and this would cause the failure of the *whole* repetition. Below we rewrite the previous grammar to indicate an error when there is no comma after an identifer. Before tyring to match a comma, we check if we have reached the end of input: ```lua local m = require'lpeglabel' local re = require'relabel' local terror = { ErrId = "expecting an identifier", ErrComma = "expecting ','", fail = "undefined" } local id = m.R'az'^1 local g = m.P{ 'S', S = m.V'List', List = m.V'Id' * (#m.P(1) * m.V'Comma' * (m.V'Id' + m.T'ErrId'))^0, Id = m.V'Sp' * id, Comma = m.V'Sp' * ',' + m.T'ErrComma', Sp = m.S' \n\t'^0, } function mymatch (g, s) local r, e, pos = g:match(s) if not r then local line, col = re.calcline(s, pos) local msg = "Error at line " .. line .. " (col " .. col .. "): " return r, msg .. terror[e] .. " before '" .. s:sub(pos) .. "'" end return r end print(mymatch(g, "one,two")) --> 8 print(mymatch(g, "one two")) --> nil Error at line 1 (col 4): expecting ',' before ' two' print(mymatch(g, "one,\n two,\nthree,4")) --> nil Error at line 3 (col 7): expecting an identifier before '4' print(mymatch(g, " 1,2")) --> nil Error at line 1 (col 2): undefined before '1,2' ``` #### Error Recovery We can specify a recovery rule that should be matched when a label is thrown. After matching the recovery rule, and possibly recording the error, the parser will resume the regular matching. The recovery rule must have the same name (or number) of the label that was thrown. For example, in the example below we expect to match rule *A*, but when a failure occur the label `Err` is thrown and then we will try to match rule *Err*: ```lua local m = require'lpeglabel' local recp = m.P"oast" local g = m.P{ 'S', S = m.V'A' * '.', A = m.P't' * (m.P'est' + m.T'Err'), Err = m.P'oast' } print(g:match("test.")) --> 6 print(g:match("toast.")) --> 7 print(g:match("oast.")) --> nil fail oast. print(g:match("toward.")) --> nil fail ward. ``` When trying to match subject 'toast.', in rule *A* the first 't' is matched, then the matching of `m.P"est"` fails and label `Err` is thrown, with the associated inpux suffix 'oast.'. The recovery rule *Err* successfully matches 'oast', so the regular matching continues, and pattern `'.'` matches the rest of the input. When matching subject 'oast.', pattern `m.P"t"` fails, and the result of the matching is nil, fail, 1. When matching 'toward.', label `Err` is thrown after matching 't', with the associated input suffix 'oward.'. As the matching of the recovery pattern fails, the result is nil, fail, 3. Usually, the recovery pattern is an expression that does not fail. In the previous example, we could have used `(m.P(1) - m.P".")^0` as the recovery pattern. Below we rewrite the grammar that describes a list of identifiers to use a recovery strategy, with the help of some auxiliary functions. Function `record`, plus function `recorderror`, will help us to save the input position where a label was thrown, while function `sync` will give us a synchronization pattern, that consumes the input while it is not possible to match a given pattern `p`. When the matching of an identifier fails, a defaul value ('NONE') is provided. ```lua local m = require'lpeglabel' local re = require'relabel' local terror = { ErrId = "expecting an identifier", ErrComma = "expecting ','", ErrList = "expecting a list of identifiers", fail = "undefined" } local subject, errors function recorderror(pos, lab) local line, col = re.calcline(subject, pos) table.insert(errors, { line = line, col = col, msg = terror[lab] }) end function record (lab) return (m.Cp() * m.Cc(lab)) / recorderror end function sync (p) return (-p * m.P(1))^0 end function defaultValue () return m.Cc"NONE" end local id = m.R'az'^1 local g = m.P{ "S", S = m.V"List" + (m.P(1) * m.T'ErrList'), List = m.V'Id' * (#m.P(1) * m.V'Comma' * (m.V'Id' + m.T'ErrId'))^0, Id = m.V'Sp' * m.C(id), Comma = m.V'Sp' * ',' + m.T'ErrComma', Sp = m.S' \n\t'^0, ErrId = record'ErrId' * sync(m.P",") * defaultValue(), ErrComma = record'ErrComma' * sync(id), ErrList = record'ErrList' * sync(m.P(-1)) * defaultValue() } function mymatch (g, s) errors = {} subject = s io.write("Input: ", s, "\n") local r = { g:match(s) } io.write("Captures (separated by ';'): ") for k, v in pairs(r) do io.write(v .. "; ") end io.write("\nSyntactic errors found: " .. #errors) if #errors > 0 then io.write("\n") local out = {} for i, err in ipairs(errors) do local msg = "Error at line " .. err.line .. " (col " .. err.col .. "): " .. err.msg table.insert(out, msg) end io.write(table.concat(out, "\n")) end print("\n") return r end mymatch(g, "one,two") --> Captures (separated by ';'): one; two; --> Syntactic errors found: 0 mymatch(g, "one two three") --> Captures (separated by ';'): one; two; three; --> Syntactic errors found: 2 --> Error at line 1 (col 4): expecting ',' --> Error at line 1 (col 8): expecting ',' mymatch(g, "1,\n two, \n3,") --> Captures (separated by ';'): NONE; --> Syntactic errors found: 1 --> Error at line 1 (col 2): expecting a list of identifiers mymatch(g, "one\n two123, \nthree,") --> Captures (separated by ';'): one; two; three; NONE; --> Syntactic errors found: 3 --> Error at line 2 (col 1): expecting ',' --> Error at line 2 (col 5): expecting ',' --> Error at line 3 (col 6): expecting an identifier ``` ##### *relabel* syntax Below we write a grammar for a simple programming language using the syntax supported by *relabel*, where `%{}` is the throw operator, and the syntax `p^l` is syntatic sugar for `p / %{l}` (given that *l* is a valid identifier name): ```lua local re = require 'relabel' local terror = { cmdSeq = "Missing ';' in CmdSeq", ifExp = "Error in expresion of 'if'", ifThen = "Error matching 'then' keyword", ifThenCmdSeq = "Error matching CmdSeq of 'then' branch", ifElseCmdSeq = "Error matching CmdSeq of 'else' branch", ifEnd = "Error matching 'end' keyword of 'if'", repeatCmdSeq = "Error matching CmdSeq of 'repeat'", repeatUntil = "Error matching 'until' keyword", repeatExp = "Error matching expression of 'until'", assignOp = "Error matching ':='", assignExp = "Error matching expression of assignment", readName = "Error matching 'NAME' after 'read'", writeExp = "Error matching expression after 'write'", simpleExp = "Error matching 'SimpleExp'", term = "Error matching 'Term'", factor = "Error matching 'Factor'", openParExp = "Error matching expression after '('", closePar = "Error matching ')'", eof = "Error, expecting EOF", undefined = "Undefined Error" } g = re.compile([[ Tiny <- CmdSeq (!. / %{eof}) CmdSeq <- (Cmd SEMICOLON^cmdSeq) (Cmd SEMICOLON^cmdSeq)* Cmd <- IfCmd / RepeatCmd / ReadCmd / WriteCmd / AssignCmd IfCmd <- IF Exp^ifExp THEN^ifThen CmdSeq^ifThenCmdSeq (ELSE CmdSeq^ifElseCmdSeq / '') END^ifEnd RepeatCmd <- REPEAT CmdSeq^repeatCmdSeq UNTIL^repeatUntil Exp^repeatExp AssignCmd <- NAME ASSIGNMENT^assignOp Exp^assignExp ReadCmd <- READ NAME^readName WriteCmd <- WRITE Exp^writeExp Exp <- SimpleExp ((LESS / EQUAL) SimpleExp^simpleExp / '') SimpleExp <- Term ((ADD / SUB) Term^term)* Term <- Factor ((MUL / DIV) Factor^factor)* Factor <- OPENPAR Exp^openParExp CLOSEPAR^closePar / NUMBER / NAME ADD <- Sp '+' ASSIGNMENT <- Sp ':=' CLOSEPAR <- Sp ')' DIV <- Sp '/' IF <- Sp 'if' ELSE <- Sp 'else' END <- Sp 'end' EQUAL <- Sp '=' LESS <- Sp '<' MUL <- Sp '*' NAME <- !RESERVED Sp [a-z]+ NUMBER <- Sp [0-9]+ OPENPAR <- Sp '(' READ <- Sp 'read' REPEAT <- Sp 'repeat' SEMICOLON <- Sp ';' SUB <- Sp '-' THEN <- Sp 'then' UNTIL <- Sp 'until' WRITE <- Sp 'write' RESERVED <- (IF / ELSE / END / READ / REPEAT / THEN / UNTIL / WRITE) ![a-z]+ Sp <- (%s / %nl)* ]], terror) local function mymatch(g, s) local r, e, pos = g:match(s) if not r then local line, col = re.calcline(s, pos) local msg = "Error at line " .. line .. " (col " .. col .. "): " return r, msg .. terror[e] end return r end local s = [[ n := 5; f := 1; repeat f := f + n; n := n - 1 until (n < 1); write f;]] print(mymatch(g, s)) --> nil Error at line 6 (col 1): Missing ';' in CmdSeq print(mymatch(g, "a : 2")) --> nil Error at line 1 (col 2): Error matching ':=' print(mymatch(g, "a := 2; 6")) --> nil Error at line 1 (col 8): Error, expecting EOF ``` ### Caveats Does not use the number **1** to specify a recovery rule, since that the entry with index 1 in the grammar table defines its initial rule. In case your grammar has many regular and recovery rules, you may get an error message such as grammar: has too many rules. In this case, we need to change *MAXRULES* in `lpltypes.h`.