222 lines
9.4 KiB
Markdown
222 lines
9.4 KiB
Markdown
goparsify [](https://circleci.com/gh/Vektah/goparsify/tree/master) [](https://godoc.org/github.com/Vektah/goparsify)
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=========
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A parser-combinator library for building easy to test, read and maintain parsers using functional composition.
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### benchmarks
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I dont have many benchmarks set up yet, but the json parser is very promising. Nearly keeping up with the stdlib for raw speed:
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```
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$ go test -bench=. -benchtime=2s -benchmem ./json
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BenchmarkUnmarshalParsec-8 20000 65682 ns/op 50460 B/op 1318 allocs/op
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BenchmarkUnmarshalParsify-8 30000 51292 ns/op 45104 B/op 334 allocs/op
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BenchmarkUnmarshalStdlib-8 30000 46522 ns/op 13953 B/op 262 allocs/op
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PASS
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ok github.com/vektah/goparsify/json 10.840s
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```
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### debugging parsers
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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.
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1. First build with debug using `-tags debug`
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2. enable logging by passing a runtime flag -parselogs or calling `EnableLogging(os.Stdout)` in your code.
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This works great with tests, eg in the goparsify source tree
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```
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$ cd html
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$ go test -tags debug -parselogs
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html.go:50 | <body>hello <p | | tag
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html.go:45 | <body>hello <p | | tstart
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html.go:45 | body>hello <p c | < | <
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html.go:20 | >hello <p color | body | identifier
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html.go:35 | >hello <p color | | attrs
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html.go:34 | >hello <p color | | attr
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html.go:20 | >hello <p color | fail | identifier
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html.go:45 | hello <p color= | > | >
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html.go:26 | hello <p color= | | elements
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html.go:25 | hello <p color= | | element
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html.go:21 | <p color="blue" | hello | text
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html.go:25 | <p color="blue" | | element
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html.go:21 | <p color="blue" | fail | text
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html.go:50 | <p color="blue" | | tag
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html.go:45 | <p color="blue" | | tstart
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html.go:45 | p color="blue"> | < | <
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html.go:20 | color="blue">w | p | identifier
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html.go:35 | color="blue">w | | attrs
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html.go:34 | color="blue">w | | attr
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html.go:20 | ="blue">world</ | color | identifier
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html.go:34 | "blue">world</p | = | =
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html.go:34 | >world</p></bod | | string literal
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html.go:34 | >world</p></bod | | attr
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html.go:20 | >world</p></bod | fail | identifier
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html.go:45 | world</p></body | > | >
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html.go:26 | world</p></body | | elements
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html.go:25 | world</p></body | | element
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html.go:21 | </p></body> | world | text
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html.go:25 | </p></body> | | element
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html.go:21 | </p></body> | fail | text
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html.go:50 | </p></body> | | tag
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html.go:45 | </p></body> | | tstart
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html.go:45 | /p></body> | < | <
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html.go:20 | /p></body> | fail | identifier
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html.go:46 | </p></body> | | tend
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html.go:46 | p></body> | </ | </
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html.go:20 | ></body> | p | identifier
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html.go:46 | </body> | > | >
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html.go:25 | </body> | | element
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html.go:21 | </body> | fail | text
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html.go:50 | </body> | | tag
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html.go:45 | </body> | | tstart
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html.go:45 | /body> | < | <
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html.go:20 | /body> | fail | identifier
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html.go:46 | </body> | | tend
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html.go:46 | body> | </ | </
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html.go:20 | > | body | identifier
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html.go:46 | | > | >
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PASS
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ok github.com/vektah/goparsify/html 0.118s
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```
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### debugging performance
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If you build the parser with -tags debug it will instrument each parser and a call to DumpDebugStats() will show stats:
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```
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Any() 415.7136ms 87000 calls json.go:35
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Map() 309.6569ms 12000 calls json.go:31
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Seq() 298.6519ms 12000 calls json.go:23
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Some() 290.6462ms 12000 calls json.go:13
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Seq() 272.6392ms 81000 calls json.go:13
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Seq() 78.0404ms 13000 calls json.go:15
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Map() 78.0404ms 13000 calls json.go:21
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Some() 77.0401ms 1000 calls json.go:15
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string literal 7.5053ms 81000 calls json.go:13
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string literal 4.5031ms 84000 calls json.go:11
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, 4.0008ms 81000 calls json.go:13
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false 2.0018ms 85000 calls json.go:10
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null 2.0005ms 87000 calls json.go:8
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true 1.501ms 87000 calls json.go:9
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: 500.8µs 81000 calls json.go:13
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[ 0s 13000 calls json.go:15
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} 0s 12000 calls json.go:23
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{ 0s 12000 calls json.go:23
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number literal 0s 31000 calls json.go:12
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] 0s 1000 calls json.go:15
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Nil 0s 0 calls profile/json.go:148
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, 0s 5000 calls json.go:15
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```
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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.
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This is **free** when the debug tag isnt used.
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### example calculator
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Lets say we wanted to build a calculator that could take an expression and calculate the result.
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Lets start with test:
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```go
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func TestNumbers(t *testing.T) {
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result, err := Calc(`1`)
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require.NoError(t, err)
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require.EqualValues(t, 1, result)
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}
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```
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Then define a parser for numbers
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```go
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var number = Map(NumberLit(), func(n Result) Result {
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switch i := n.Result.(type) {
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case int64:
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return Result{Result: float64(i)}
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case float64:
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return Result{Result: i}
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default:
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panic(fmt.Errorf("unknown value %#v", i))
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}
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})
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func Calc(input string) (float64, error) {
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result, err := Run(y, input)
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if err != nil {
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return 0, err
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}
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return result.(float64), nil
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}
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```
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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.
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Run the tests and make sure everything is ok.
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Time to add addition
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```go
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func TestAddition(t *testing.T) {
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result, err := Calc(`1+1`)
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require.NoError(t, err)
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require.EqualValues(t, 2, result)
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}
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var sumOp = Chars("+-", 1, 1)
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sum = Map(Seq(number, Some(And(sumOp, number))), func(n Result) Result {
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i := n.Child[0].Result.(float64)
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for _, op := range n.Child[1].Child {
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switch op.Child[0].Token {
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case "+":
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i += op.Child[1].Result.(float64)
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case "-":
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i -= op.Child[1].Result.(float64)
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}
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}
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return Result{Result: i}
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})
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// and update Calc to point to the new root parser -> `result, err := ParseString(sum, input)`
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```
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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.
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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:
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```
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typechecking loop involving value = goparsify.Any(number, groupExpr)
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```
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we need to break the loop using a pointer, then set its value in init
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```go
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var (
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value Parser
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prod = Seq(&value, Some(And(prodOp, &value)))
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)
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func init() {
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value = Any(number, groupExpr)
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}
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```
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Take a look at [calc](calc/calc.go) for a full example.
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### preventing backtracking with cuts
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A cut is a marker that prevents backtracking past the point it was set. This greatly improves error messages when used correctly:
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```go
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alpha := Chars("a-z")
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// without a cut if the close tag is left out the parser will backtrack and ignore the rest of the string
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nocut := Many(Any(Seq("<", alpha, ">"), alpha))
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_, err := Run(nocut, "asdf <foo")
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fmt.Println(err.Error())
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// Outputs: left unparsed: <foo
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// 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
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cut := Many(Any(Seq("<", Cut, alpha, ">"), alpha))
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_, err = Run(cut, "asdf <foo")
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fmt.Println(err.Error())
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// Outputs: offset 9: expected >
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```
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### prior art
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Inspired by https://github.com/prataprc/goparsec
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