sourceafPegger::Rule.fan

** (Advanced)  
** Models a PEG Rule.
** 
** Rules are commonly created by using methods from the `Rules` mixin, but may be parsed from string patterns.
** 
** `Rule` *may* sub-classed to create your own custom rules. 
@Js
abstract class Rule {
    
    ** Creates a rule by parsing the given pattern:
    ** 
    **   syntax: fantom 
    **   Rule.fromPattern("[abc] / [xyz]")
    ** 
    ** See `Peg.parseRule`
    static new parseRule(Str pattern) {
        PegGrammar().parseRule(pattern)
    }

    ** The name of this rule. Only rules with names appear in debug output.
    ** Should be legal Fantom identifier (think variable names!).
    virtual Str? name {
        set {
            if (!it.chars.first.isAlpha || it.chars.any { !it.isAlphaNum && it != '_' && it != '-' })
                throw ArgErr("Name must be a valid Fantom identifier: $it")
            &name = it
        }
    }
    
    ** A label for this rule.
    virtual Str? label {
        set {
            if (!it.chars.first.isAlpha || it.chars.any { !it.isAlphaNum && it != '_' && it != '-' })
                throw ArgErr("Label must be a valid Fantom identifier: $it")
            &label = it
        }
    }
    
    ** Disable debugging of this rule if it gets too noisy.
    virtual Bool debug          := true

    ** Not all rules are useful in the parsed AST. 
    virtual Bool useInResult    := true

    ** Override to implement Rule logic.
    @NoDoc
    abstract Bool doProcess(RuleCtx ctx)
    
    ** Returns the PEG expression for this rule. Example:
    ** 
    **   [a-zA-Z0-9]
    abstract Str expression()

    ** Returns the PEG definition for this rule. Example:
    ** 
    **   alphaNum <- [a-zA-Z0-9]
    Str definition() {
        if (name == null) return expression
        ex := name
        if (!useInResult)   ex = "-" + ex
        if (!debug)         ex = ex + "-"
        return "${ex} <- ${expression}"
    }

    ** A helpful builder method for setting the name.
    This withName(Str name) {
        this.name = name
        return this
    }

    ** A helpful builder method for setting the label.
    This withLabel(Str label) {
        this.label = label
        return this
    }

    ** A helpful builder method for turning debug off.
    This debugOff() {
        this.debug = false
        return this
    }
    
    ** A helpful builder method for removing this rule from tree results.
    This excludeFromResults() {
        this.useInResult = false
        return this
    }
    
    ** Matches this rule against the given string.
    ** 
    ** See `Peg.match`
    Match? match(Str str) {
        Peg(str, this).match
    }
    
    @NoDoc
    virtual Str typeName() {
        // if converting doProcess() to processFn then THIS method will need to be converted to another field 
        name := typeof.name.decapitalize
        return name.endsWith("Rule") ? name[0..<-4] : name
    }

    @Operator @NoDoc
    virtual This add(Rule rule) {
        throw Err("${typeof.qname} does not support add()")
    }
    
    @NoDoc
    internal Str _dis(Bool inner := false) {
        dis := inner && name == null && (this is SequenceRule || this is FirstOfRule)
            ? "(" + expression + ")"
            : (name ?: expression)
        return label != null ? "${label}:${dis}" : dis
    }
    
    @NoDoc
    override Str toStr() { _dis }  
}