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-rw-r--r--combinator.go12
-rw-r--r--combinator_test.go13
-rw-r--r--debugon.go2
-rw-r--r--examples_test.go24
-rw-r--r--html/html.go8
-rw-r--r--readme.md23
-rw-r--r--state.go2
-rw-r--r--state_test.go6
8 files changed, 74 insertions, 16 deletions
diff --git a/combinator.go b/combinator.go
index 7a811bc..1572e6c 100644
--- a/combinator.go
+++ b/combinator.go
@@ -45,13 +45,12 @@ func Any(parsers ...Parserish) Parser {
for _, parser := range parserfied {
node := parser(ps)
if ps.Errored() {
- if ps.Cut > startpos {
- longestError = ps.Error
- break
- }
if ps.Error.pos > longestError.pos {
longestError = ps.Error
}
+ if ps.Cut > startpos {
+ break
+ }
ps.Recover()
continue
}
@@ -91,7 +90,7 @@ func manyImpl(min int, op Parserish, sep ...Parserish) Parser {
for {
node := opParser(ps)
if ps.Errored() {
- if len(result.Child) < min {
+ if len(result.Child) < min || ps.Cut > ps.Pos {
ps.Pos = startpos
return result
}
@@ -116,8 +115,9 @@ func Maybe(parser Parserish) Parser {
parserfied := Parsify(parser)
return NewParser("Maybe()", func(ps *State) Result {
+ startpos := ps.Pos
node := parserfied(ps)
- if ps.Errored() {
+ if ps.Errored() && ps.Cut <= startpos {
ps.Recover()
}
diff --git a/combinator_test.go b/combinator_test.go
index 5ac227f..acf0e84 100644
--- a/combinator_test.go
+++ b/combinator_test.go
@@ -163,12 +163,23 @@ func TestBind(t *testing.T) {
}
func TestCut(t *testing.T) {
- // does backtracking happen anywhere else?
t.Run("test any", func(t *testing.T) {
_, ps := runParser("var world", Any(Seq("var", Cut, "hello"), "var world"))
require.Equal(t, "offset 4: expected hello", ps.Error.Error())
require.Equal(t, 0, ps.Pos)
})
+
+ t.Run("test many", func(t *testing.T) {
+ _, ps := runParser("hello <world", Many(Any(Seq("<", Cut, Chars("a-z"), ">"), Chars("a-z"))))
+ require.Equal(t, "offset 12: expected >", ps.Error.Error())
+ require.Equal(t, 0, ps.Pos)
+ })
+
+ t.Run("test maybe", func(t *testing.T) {
+ _, ps := runParser("var", Maybe(Seq("var", Cut, "hello")))
+ require.Equal(t, "offset 3: expected hello", ps.Error.Error())
+ require.Equal(t, 0, ps.Pos)
+ })
}
func TestMerge(t *testing.T) {
diff --git a/debugon.go b/debugon.go
index 4e68db3..9154472 100644
--- a/debugon.go
+++ b/debugon.go
@@ -32,7 +32,7 @@ type debugParser struct {
}
func (dp *debugParser) Name() string {
- if len(activeParsers) > 2 && activeParsers[len(activeParsers)-2].Var == dp.Var {
+ if len(activeParsers) > 1 && activeParsers[len(activeParsers)-2].Var == dp.Var {
return dp.Match
}
return dp.Var
diff --git a/examples_test.go b/examples_test.go
new file mode 100644
index 0000000..a1de129
--- /dev/null
+++ b/examples_test.go
@@ -0,0 +1,24 @@
+package goparsify_test
+
+import (
+ "fmt"
+
+ . "github.com/vektah/goparsify"
+)
+
+func ExampleCuts() {
+ // without a cut if the close tag is left out the parser will backtrack and ignore the rest of the string
+ alpha := Chars("a-z")
+ nocut := Many(Any(Seq("<", alpha, ">"), alpha))
+ _, err := Run(nocut, "asdf <foo")
+ fmt.Println(err.Error())
+
+ // 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())
+
+ // Output:
+ // left unparsed: <foo
+ // offset 9: expected >
+}
diff --git a/html/html.go b/html/html.go
index 0def587..e2dff0b 100644
--- a/html/html.go
+++ b/html/html.go
@@ -42,17 +42,17 @@ var (
return Result{Result: attr}
})
- tstart = Seq("<", Cut, identifier, attrs, ">")
+ tstart = Seq("<", identifier, Cut, attrs, ">")
tend = Seq("</", Cut, identifier, ">")
)
func init() {
- tag = Map(Seq(tstart, elements, tend), func(node Result) Result {
+ tag = Map(Seq(tstart, Cut, elements, tend), func(node Result) Result {
openTag := node.Child[0]
return Result{Result: Tag{
- Name: openTag.Child[2].Token,
+ Name: openTag.Child[1].Token,
Attributes: openTag.Child[3].Result.(map[string]string),
- Body: node.Child[1].Result.([]interface{}),
+ Body: node.Child[2].Result.([]interface{}),
}}
})
diff --git a/readme.md b/readme.md
index 9e712b0..1da05ed 100644
--- a/readme.md
+++ b/readme.md
@@ -7,9 +7,9 @@ A parser-combinator library for building easy to test, read and maintain parsers
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
+BenchmarkUnmarshalParsec-8 20000 65682 ns/op 50460 B/op 1318 allocs/op
+BenchmarkUnmarshalParsify-8 30000 51292 ns/op 45104 B/op 334 allocs/op
+BenchmarkUnmarshalStdlib-8 30000 46522 ns/op 13953 B/op 262 allocs/op
PASS
ok github.com/vektah/goparsify/json 10.840s
```
@@ -198,6 +198,23 @@ func init() {
Take a look at [calc](calc/calc.go) 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:
+```go
+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
diff --git a/state.go b/state.go
index 384eb32..64737dc 100644
--- a/state.go
+++ b/state.go
@@ -92,7 +92,7 @@ func (s *State) Get() string {
// Preview of the the next x characters
func (s *State) Preview(x int) string {
- if s.Pos > len(s.Input) {
+ if s.Pos >= len(s.Input) {
return ""
}
if len(s.Input)-s.Pos >= x {
diff --git a/state_test.go b/state_test.go
index 5ecf367..c279d14 100644
--- a/state_test.go
+++ b/state_test.go
@@ -43,3 +43,9 @@ func TestState_Errors(t *testing.T) {
require.Equal(t, 2, ps.Error.Pos())
require.True(t, ps.Errored())
}
+
+func TestState_Preview(t *testing.T) {
+ require.Equal(t, "", NewState("").Preview(10))
+ require.Equal(t, "asdf", NewState("asdf").Preview(10))
+ require.Equal(t, "asdfasdfas", NewState("asdfasdfasdf").Preview(10))
+}
generated by cgit v1.2.3 (git 2.39.1) at 2025-09-11 08:51:11 +0000