[FTR] Grammar test suite

[romshark]

Proposal

Provide a simple test-helper function like llparser.TestGrammar(t *testing.T, mainRule *llparser.Rule) which automatically performs several semantic tests on the specified grammar. The user would only need to wrap it this way:

// TestFoolangGrammar tests the grammar of the Foo language for semantic errors
func TestFoolangGrammar(t *testing.T) {
  llparser.TestGrammar(t, foolang.Grammar())
}

Semantic grammar errors

• Duplicate patterns in Either
• Nested Optional, OneOrMore and ZeroOrMore
• Empty Sequence
• Endlessly recursive Rule
• Fn with nil value in Checked and Lexed
• Empty Expectation in TermExact
• etc.

Current Workaround

Currently, there's no simple way to check a grammar for potential errors.

[romshark]

This test should probably be done automatically in the Parser constructor function to prevent malformed grammar from being accepted by llparser.NewParser. Following changes would be necessary:

type Parser struct{
  rule *Rule
  errRule *Rule
}

func NewParser(rule *Rule, errRule *Rule) Parser

func (pr Parser) Parse(source *SourceFile) (Fragment, error)