expression.go

package eval

import "reflect"

type (
	// Expression built by the engine through the DSL functions.
	Expression interface {
		// EvalName is the qualified name of the DSL expression e.g. "service
		// bottle".
		EvalName() string
	}

	// A Root expression represents an entry point to the executed DSL: upon
	// execution the DSL engine iterates over all root expressions and calls
	// their WalkSets methods to iterate over the sub-expressions.
	Root interface {
		Expression
		// WalkSets implements the visitor pattern: is is called by the engine so
		// the DSL can control the order of execution. WalkSets calls back the
		// engine via the given iterator as many times as needed providing the
		// expression sets on each callback.
		WalkSets(SetWalker)
		// DependsOn returns the list of other DSL roots this root depends on. The
		// engine uses this function to order the execution of the DSL roots.
		DependsOn() []Root
		// Packages returns the import path to the Go packages that make up the
		// DSL. This is used to skip frames that point to files in these packages
		// when computing the location of errors.
		Packages() []string
	}

	// A Source expression embeds DSL to be executed after the process is loaded.
	Source interface {
		// DSL returns the DSL used to initialize the expression in a second pass.
		DSL() func()
	}

	// A Preparer expression requires an additional pass after the DSL has
	// executed and BEFORE it is validated (e.g. to flatten inheritance)
	Preparer interface {
		// Prepare is run by the engine right after the DSL has run. Prepare
		// cannot fail, any potential failure should be returned by implementing
		// Validator instead.
		Prepare()
	}

	// A Validator expression can be validated.
	Validator interface {
		// Validate runs after Prepare if the expression is a Preparer.  It returns
		// nil if the expression contains no validation error. The Validate
		// implementation may take advantage of ValidationErrors to report more
		// than one errors at a time.
		Validate() error
	}

	// A Finalizer expression requires an additional pass after the DSL has
	// executed and has been validated (e.g. to merge generated expressions or
	// initialize default values).
	Finalizer interface {
		// Finalize is run by the engine as the last step. Finalize cannot fail,
		// any potential failure should be returned by implementing Validator
		// instead.
		Finalize()
	}

	// DSLFunc is a type that DSL expressions may embed to store DSL. It
	// implements Source.
	DSLFunc func()

	// TopExpr is the type of Top.
	TopExpr string

	// ExpressionSet is a sequence of expressions processed in order. Each DSL
	// implementation provides an arbitrary number of expression sets to the
	// engine via iterators (see the Root interface WalkSets method).
	//
	// The items in the set may implement the Source, Preparer, Validator and/or
	// Finalizer interfaces to enable the corresponding behaviors during DSL
	// execution. The engine first runs the expression DSLs (for the ones that
	// implement Source), then prepares them (for the ones that implement
	// Preparer), then validates them (for the ones that implement Validator), and
	// finalizes them (for the ones that implement Finalizer).
	ExpressionSet []Expression

	// SetWalker is the function signature used to iterate over expression sets
	// with WalkSets.
	SetWalker func(s ExpressionSet) error
)

// Top is the expression returned by Current when the execution stack is empty.
const Top TopExpr = "top-level"

// DSL returns the DSL function.
func (f DSLFunc) DSL() func() {
	return f
}

// EvalName is the name is the qualified name of the expression.
func (t TopExpr) EvalName() string { return string(t) }

// ToExpressionSet is a convenience function that accepts a slice of expressions
// and builds the corresponding ExpressionSet.
func ToExpressionSet(slice interface{}) ExpressionSet {
	if slice == nil {
		return nil
	}
	s := reflect.ValueOf(slice)
	if s.Kind() != reflect.Slice {
		panic("ToExpressionSet() given a non-slice type") // bug
	}
	ret := make(ExpressionSet, s.Len())
	for i := 0; i < s.Len(); i++ {
		ret[i] = s.Index(i).Interface().(Expression)
	}

	return ret
}