| Function | Description |
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 |
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`.