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goparsify [](https://circleci.com/gh/Vektah/goparsify/tree/master) [](https://godoc.org/github.com/Vektah/goparsify)
=========
A parser-combinator library for building easy to test, read and maintain parsers using functional composition.
### todo
- fatal errors: Some way for a parser to say "Ive found a good match, the input is broken, stop here with an error"
- better errors: currently only the longest error is returned, but it would be nice to show all expected tokens that could follow.
### benchmarks
I dont have many benchmarks set up yet, but the json parser is very promising. Nearly keeping up with the stdlib for raw speed:
```
$ go test -bench=. -benchtime=2s -benchmem ./json
BenchmarkUnmarshalParsec-8 50000 71447 ns/op 50464 B/op 1318 allocs/op
BenchmarkUnmarshalParsify-8 50000 56414 ns/op 43887 B/op 334 allocs/op
BenchmarkUnmarshalStdlib-8 50000 50187 ns/op 13949 B/op 262 allocs/op
PASS
ok github.com/vektah/goparsify/json 10.840s
```
### debugging mode
If you build the parser with -tags debug it will instrument each parser and a call to DumpDebugStats() will show stats:
```
Any() 415.7136ms 87000 calls json.go:35
Map() 309.6569ms 12000 calls json.go:31
Seq() 298.6519ms 12000 calls json.go:23
Some() 290.6462ms 12000 calls json.go:13
Seq() 272.6392ms 81000 calls json.go:13
Seq() 78.0404ms 13000 calls json.go:15
Map() 78.0404ms 13000 calls json.go:21
Some() 77.0401ms 1000 calls json.go:15
string literal 7.5053ms 81000 calls json.go:13
string literal 4.5031ms 84000 calls json.go:11
, 4.0008ms 81000 calls json.go:13
false 2.0018ms 85000 calls json.go:10
null 2.0005ms 87000 calls json.go:8
true 1.501ms 87000 calls json.go:9
: 500.8µs 81000 calls json.go:13
[ 0s 13000 calls json.go:15
} 0s 12000 calls json.go:23
{ 0s 12000 calls json.go:23
number literal 0s 31000 calls json.go:12
] 0s 1000 calls json.go:15
Nil 0s 0 calls profile/json.go:148
, 0s 5000 calls json.go:15
```
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:
```go
func TestNumbers(t *testing.T) {
result, err := Calc(`1`)
require.NoError(t, err)
require.EqualValues(t, 1, result)
}
```
Then define a parser for numbers
```go
var number = Map(NumberLit(), func(n Node) Node {
switch i := n.Result.(type) {
case int64:
return Node{Result: float64(i)}
case float64:
return Node{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
```go
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 = Map(Seq(number, Some(And(sumOp, number))), func(n Node) Node {
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 Node{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
```go
var (
value Parser
prod = Seq(&value, Some(And(prodOp, &value)))
)
func init() {
value = Any(number, groupExpr)
}
```
Take a look at [calc](calc/calc.go) for a full example.
### prior art
Inspired by https://github.com/prataprc/goparsec
|
