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// Copyright 2018 The CUE Authors
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// See the License for the specific language governing permissions and
// limitations under the License.
// Package build defines data types and utilities for defining CUE configuration
// instances.
// This package enforces the rules regarding packages and instances as defined
// in the spec, but it leaves any other details, as well as handling of modules,
// up to the implementation.
// A full implementation of instance loading can be found in the loader package.
// WARNING: this packages may change. It is fine to use load and cue, who both
// use this package.
package build
import (
// A Context keeps track of state of building instances and caches work.
type Context struct {
ctxt context.Context
loader LoadFunc
parseFunc func(str string, src interface{}) (*ast.File, error)
initialized bool
imports map[string]*Instance
// NewInstance creates an instance for this Context.
func (c *Context) NewInstance(dir string, f LoadFunc) *Instance {
if c == nil {
c = &Context{}
if f == nil {
f = c.loader
return &Instance{
ctxt: c,
loadFunc: f,
Dir: dir,
// Complete finishes the initialization of an instance. All files must have
// been added with AddFile before this call.
func (inst *Instance) Complete() error {
if inst.done {
return inst.Err
inst.done = true
err := inst.complete()
if err != nil {
if inst.Err != nil {
inst.Incomplete = true
return inst.Err
return nil
func (c *Context) init() {
if !c.initialized {
c.initialized = true
c.ctxt = context.Background()
c.imports = map[string]*Instance{}
// Options:
// - certain parse modes
// - parallellism
// - error handler (allows cancelling the context)
// - file set.
// NewContext creates a new build context.
// All instances must be created with a context.
func NewContext(opts ...Option) *Context {
c := &Context{}
for _, o := range opts {
return c
// Option define build options.
type Option func(c *Context)
// Loader sets parsing options.
func Loader(f LoadFunc) Option {
return func(c *Context) { c.loader = f }
// ParseFile is called to read and parse each file
// when building syntax tree.
// It must be safe to call ParseFile simultaneously from multiple goroutines.
// If ParseFile is nil, the loader will uses parser.ParseFile.
// ParseFile should parse the source from src and use filename only for
// recording position information.
// An application may supply a custom implementation of ParseFile
// to change the effective file contents or the behavior of the parser,
// or to modify the syntax tree. For example, changing the backwards
// compatibility.
func ParseFile(f func(filename string, src interface{}) (*ast.File, error)) Option {
return func(c *Context) { c.parseFunc = f }