10 KiB
goparsify 
A parser-combinator library for building easy to test, read and maintain parsers using functional composition.
Everything should be unicode safe by default, but you can opt out of unicode whitespace for a decent ~20% performance boost.
Run(parser, input, ASCIIWhitespace)
benchmarks
I dont have many benchmarks set up yet, but the json parser keeps up with the stdlib for raw speed:
$ go test -bench=. -benchtime=2s -benchmem ./json
BenchmarkUnmarshalParsec-8 50000 66012 ns/op 50462 B/op 1318 allocs/op
BenchmarkUnmarshalParsify-8 100000 46713 ns/op 44543 B/op 332 allocs/op
BenchmarkUnmarshalStdlib-8 100000 46967 ns/op 13952 B/op 262 allocs/op
PASS
ok github.com/vektah/goparsify/json 14.424s
debugging parsers
When a parser isnt working as you intended you can build with debugging and enable logging to get a detailed log of exactly what the parser is doing.
- First build with debug using
-tags debug - enable logging by calling
EnableLogging(os.Stdout)in your code
This works great with tests, eg in the goparsify source tree
$ cd html
$ go test -tags debug -parselogs
html.go:50 | <body>hello <p | | tag
html.go:45 | <body>hello <p | | tstart
html.go:45 | body>hello <p c | < | <
html.go:20 | >hello <p color | body | identifier
html.go:35 | >hello <p color | | attrs
html.go:34 | >hello <p color | | attr
html.go:20 | >hello <p color | fail | identifier
html.go:45 | hello <p color= | > | >
html.go:26 | hello <p color= | | elements
html.go:25 | hello <p color= | | element
html.go:21 | <p color="blue" | hello | text
html.go:25 | <p color="blue" | | element
html.go:21 | <p color="blue" | fail | text
html.go:50 | <p color="blue" | | tag
html.go:45 | <p color="blue" | | tstart
html.go:45 | p color="blue"> | < | <
html.go:20 | color="blue">w | p | identifier
html.go:35 | color="blue">w | | attrs
html.go:34 | color="blue">w | | attr
html.go:20 | ="blue">world</ | color | identifier
html.go:34 | "blue">world</p | = | =
html.go:34 | >world</p></bod | | string literal
html.go:34 | >world</p></bod | | attr
html.go:20 | >world</p></bod | fail | identifier
html.go:45 | world</p></body | > | >
html.go:26 | world</p></body | | elements
html.go:25 | world</p></body | | element
html.go:21 | </p></body> | world | text
html.go:25 | </p></body> | | element
html.go:21 | </p></body> | fail | text
html.go:50 | </p></body> | | tag
html.go:45 | </p></body> | | tstart
html.go:45 | /p></body> | < | <
html.go:20 | /p></body> | fail | identifier
html.go:46 | </p></body> | | tend
html.go:46 | p></body> | </ | </
html.go:20 | ></body> | p | identifier
html.go:46 | </body> | > | >
html.go:25 | </body> | | element
html.go:21 | </body> | fail | text
html.go:50 | </body> | | tag
html.go:45 | </body> | | tstart
html.go:45 | /body> | < | <
html.go:20 | /body> | fail | identifier
html.go:46 | </body> | | tend
html.go:46 | body> | </ | </
html.go:20 | > | body | identifier
html.go:46 | | > | >
PASS
ok github.com/vektah/goparsify/html 0.118s
debugging performance
If you build the parser with -tags debug it will instrument each parser and a call to DumpDebugStats() will show stats:
var name matches total time self time calls errors location
_value Any() 5.1725682s 57.0243ms 878801 calls 0 errors json.go:36
_properties string literal 131.5662ms 45.0273ms 818185 calls 0 errors json.go:14
_properties Seq() 3.579274s 42.016ms 818185 calls 0 errors json.go:14
_properties , 50.5254ms 35.5182ms 818185 calls 121213 errors json.go:14
_properties : 51.5256ms 35.0183ms 818185 calls 0 errors json.go:14
_string string literal 78.0462ms 28.0172ms 671723 calls 136369 errors json.go:12
_number number literal 34.5187ms 15.5065ms 287886 calls 106066 errors json.go:13
_null null 17.011ms 8.5058ms 252538 calls 252535 errors json.go:9
_properties Some() 3.7588588s 7.5023ms 121213 calls 0 errors json.go:14
_object { 10.5049ms 7.0029ms 161616 calls 40403 errors json.go:24
_true true 10.0072ms 6.505ms 252537 calls 232332 errors json.go:10
_false false 9.0039ms 4.5032ms 232333 calls 222229 errors json.go:11
_object Seq() 3.81739s 4.5016ms 161616 calls 40403 errors json.go:24
_array [ 5.0013ms 4.0011ms 65660 calls 55558 errors json.go:16
_object } 5.5023ms 2.5021ms 121213 calls 0 errors json.go:24
_array , 2.0018ms 1.5026ms 50509 calls 10102 errors json.go:16
_array Some() 933.4591ms 500.8µs 10102 calls 0 errors json.go:16
_array Seq() 952.9664ms 0s 65660 calls 55558 errors json.go:16
_array ] 0s 0s 10102 calls 0 errors json.go:16
All times are cumulative, it would be nice to break this down into a parse tree with relative times. This is a nice addition to pprof as it will break down the parsers based on where they are used instead of grouping them all by type.
This is free when the debug tag isnt used.
example calculator
Lets say we wanted to build a calculator that could take an expression and calculate the result.
Lets start with test:
func TestNumbers(t *testing.T) {
result, err := Calc(`1`)
require.NoError(t, err)
require.EqualValues(t, 1, result)
}
Then define a parser for numbers
var number = NumberLit().Map(func(n Result) Result {
switch i := n.Result.(type) {
case int64:
return Result{Result: float64(i)}
case float64:
return Result{Result: i}
default:
panic(fmt.Errorf("unknown value %#v", i))
}
})
func Calc(input string) (float64, error) {
result, err := Run(y, input)
if err != nil {
return 0, err
}
return result.(float64), nil
}
This parser will return numbers either as float64 or int depending on the literal, for this calculator we only want floats so we Map the results and type cast.
Run the tests and make sure everything is ok.
Time to add addition
func TestAddition(t *testing.T) {
result, err := Calc(`1+1`)
require.NoError(t, err)
require.EqualValues(t, 2, result)
}
var sumOp = Chars("+-", 1, 1)
sum = Seq(number, Some(And(sumOp, number))).Map(func(n Result) Result {
i := n.Child[0].Result.(float64)
for _, op := range n.Child[1].Child {
switch op.Child[0].Token {
case "+":
i += op.Child[1].Result.(float64)
case "-":
i -= op.Child[1].Result.(float64)
}
}
return Result{Result: i}
})
// and update Calc to point to the new root parser -> `result, err := ParseString(sum, input)`
This parser will match number ([+-] number)+, then map its to be the sum. See how the Child map directly to the positions in the parsers? n is the result of the and, n.Child[0] is its first argument, n.Child[1] is the result of the Some parser, n.Child[1].Child[0] is the result of the first And and so fourth. Given how closely tied the parser and the Map are it is good to keep the two together.
You can continue like this and add multiplication and parenthesis fairly easily. Eventually if you keep adding parsers you will end up with a loop, and go will give you a handy error message like:
typechecking loop involving value = goparsify.Any(number, groupExpr)
we need to break the loop using a pointer, then set its value in init
var (
value Parser
prod = Seq(&value, Some(And(prodOp, &value)))
)
func init() {
value = Any(number, groupExpr)
}
Take a look at calc for a full example.
preventing backtracking with cuts
A cut is a marker that prevents backtracking past the point it was set. This greatly improves error messages when used correctly:
alpha := Chars("a-z")
// without a cut if the close tag is left out the parser will backtrack and ignore the rest of the string
nocut := Many(Any(Seq("<", alpha, ">"), alpha))
_, err := Run(nocut, "asdf <foo")
fmt.Println(err.Error())
// Outputs: left unparsed: <foo
// with a cut, once we see the open tag we know there must be a close tag that matches it, so the parser will error
cut := Many(Any(Seq("<", Cut(), alpha, ">"), alpha))
_, err = Run(cut, "asdf <foo")
fmt.Println(err.Error())
// Outputs: offset 9: expected >
prior art
Inspired by https://github.com/prataprc/goparsec