// FnForm = 'fn' StringPrimary LambdaPrimary
//
// fn f []{foobar} is a shorthand for set '&'f = []{foobar}.
func compileFn(cp *compiler, fn *parse.Form) op {
	if len(fn.Args) == 0 {
		cp.errorf(fn.End(), "should be followed by function name")
	}
	fnName := mustString(cp, fn.Args[0], "must be a literal string")
	varName := FnPrefix + fnName

	if len(fn.Args) == 1 {
		cp.errorf(fn.Args[0].End(), "should be followed by a lambda")
	}
	pn := mustPrimary(cp, fn.Args[1], "should be a lambda")
	if pn.Type != parse.Lambda {
		cp.errorf(pn.Begin(), "should be a lambda")
	}
	if len(fn.Args) > 2 {
		cp.errorf(fn.Args[2].Begin(), "superfluous argument")
	}

	cp.registerVariableSet(":" + varName)
	op := cp.lambda(pn)

	return func(ec *evalCtx) {
		closure := op(ec)[0].(*Closure)
		closure.Op = makeFnOp(closure.Op)
		ec.local[varName] = newPtrVariable(closure)
	}
}

// FnForm = 'fn' StringPrimary LambdaPrimary
//
// fn f []{foobar} is a shorthand for set '&'f = []{foobar}.
func compileFn(cp *compiler, fn *parse.Form) OpFunc {
	if len(fn.Args) == 0 {
		end := fn.End()
		cp.errorpf(end, end, "should be followed by function name")
	}
	fnName := mustString(cp, fn.Args[0], "must be a literal string")
	varName := FnPrefix + fnName

	if len(fn.Args) == 1 {
		end := fn.Args[0].End()
		cp.errorpf(end, end, "should be followed by a lambda")
	}
	pn := mustPrimary(cp, fn.Args[1], "should be a lambda")
	if pn.Type != parse.Lambda {
		cp.compiling(pn)
		cp.errorf("should be a lambda")
	}
	if len(fn.Args) > 2 {
		cp.errorpf(fn.Args[2].Begin(), fn.Args[len(fn.Args)-1].End(), "superfluous argument(s)")
	}

	cp.registerVariableSet(":" + varName)
	op := cp.lambda(pn)

	return func(ec *EvalCtx) {
		// Initialize the function variable with the builtin nop
		// function. This step allows the definition of recursive
		// functions; the actual function will never be called.
		ec.local[varName] = NewPtrVariable(&BuiltinFn{"<shouldn't be called>", nop})
		closure := op(ec)[0].(*Closure)
		closure.Op = makeFnOp(closure.Op)
		ec.local[varName].Set(closure)
	}
}

func (cp *compiler) form(n *parse.Form) Op {
	if len(n.Assignments) > 0 {
		if n.Head != nil {
			cp.errorf(n.Begin(), "temporary assignments not yet supported")
		}
		ops := cp.assignments(n.Assignments)
		return func(ec *EvalCtx) {
			for _, op := range ops {
				op(ec)
			}
		}
	}

	headStr, ok := oneString(n.Head)
	if ok {
		compileForm, ok := builtinSpecials[headStr]
		if ok {
			// special form
			return compileForm(cp, n)
		}
		// Ignore the output. If a matching function exists it will be
		// captured and eventually the Evaler executes it. If not, nothing
		// happens here and the Evaler executes an external command.
		cp.registerVariableGet(FnPrefix + headStr)
		// XXX Dynamic head names should always refer to external commands
	}
	headOp := cp.compound(n.Head)
	argOps := cp.compounds(n.Args)
	// TODO: n.NamedArgs
	redirOps := cp.redirs(n.Redirs)
	// TODO: n.ErrorRedir

	p := n.Begin()
	// ec here is always a subevaler created in compiler.pipeline, so it can
	// be safely modified.
	return func(ec *EvalCtx) {
		// head
		headValues := headOp(ec)
		headMust := ec.must(headValues, "the head of command", p)
		headMust.mustLen(1)
		switch headValues[0].(type) {
		case String, Caller, Indexer:
		default:
			headMust.error("a string or callable", headValues[0].Kind())
		}

		// args
		var args []Value
		for _, argOp := range argOps {
			args = append(args, argOp(ec)...)
		}

		// redirs
		for _, redirOp := range redirOps {
			redirOp(ec)
		}

		ec.resolveCaller(headValues[0]).Call(ec, args)
	}
}

func (cp *compiler) formOp(n *parse.Form) Op {
	return Op{cp.form(n), n.Begin(), n.End()}
}

func (cp *compiler) form(n *parse.Form) OpFunc {
	var saveVarsOps []LValuesOp
	var assignmentOps []Op
	if len(n.Assignments) > 0 {
		assignmentOps = cp.assignmentOps(n.Assignments)
		if n.Head == nil {
			// Permanent assignment.
			return func(ec *EvalCtx) {
				for _, op := range assignmentOps {
					op.Exec(ec)
				}
			}
		} else {
			for _, a := range n.Assignments {
				v, r := cp.lvaluesOp(a.Dst)
				saveVarsOps = append(saveVarsOps, v, r)
			}
		}
	}

	if n.Control != nil {
		if len(n.Args) > 0 {
			cp.errorpf(n.Args[0].Begin(), n.Args[len(n.Args)-1].End(), "control structure takes no arguments")
		}
		redirOps := cp.redirOps(n.Redirs)
		controlOp := cp.controlOp(n.Control)
		return func(ec *EvalCtx) {
			for _, redirOp := range redirOps {
				redirOp.Exec(ec)
			}
			controlOp.Exec(ec)
		}
	}

	headStr, ok := oneString(n.Head)
	if ok {
		compileForm, ok := builtinSpecials[headStr]
		if ok {
			// special form
			return compileForm(cp, n)
		}
		// Ignore the output. If a matching function exists it will be
		// captured and eventually the Evaler executes it. If not, nothing
		// happens here and the Evaler executes an external command.
		cp.registerVariableGet(FnPrefix + headStr)
		// XXX Dynamic head names should always refer to external commands
	}
	headOp := cp.compoundOp(n.Head)
	argOps := cp.compoundOps(n.Args)
	optsOp := cp.mapPairs(n.Opts)
	redirOps := cp.redirOps(n.Redirs)
	// TODO: n.ErrorRedir

	begin, end := n.Begin(), n.End()
	// ec here is always a subevaler created in compiler.pipeline, so it can
	// be safely modified.
	return func(ec *EvalCtx) {
		// Temporary assignment.
		if len(saveVarsOps) > 0 {
			// There is a temporary assignment.
			// Save variables.
			var saveVars []Variable
			var saveVals []Value
			for _, op := range saveVarsOps {
				saveVars = append(saveVars, op.Exec(ec)...)
			}
			for _, v := range saveVars {
				val := v.Get()
				saveVals = append(saveVals, val)
				Logger.Printf("saved %s = %s", v, val)
			}
			// Do assignment.
			for _, op := range assignmentOps {
				op.Exec(ec)
			}
			// Defer variable restoration. Will be executed even if an error
			// occurs when evaling other part of the form.
			defer func() {
				for i, v := range saveVars {
					val := saveVals[i]
					if val == nil {
						// XXX Old value is nonexistent. We should delete the
						// variable. However, since the compiler now doesn't delete
						// it, we don't delete it in the evaler either.
						val = String("")
					}
					v.Set(val)
					Logger.Printf("restored %s = %s", v, val)
				}
			}()
		}

		// head
		headValues := headOp.Exec(ec)
		ec.must(headValues, "head of command", headOp.Begin, headOp.End).mustLen(1)
		headFn := mustFn(headValues[0])

		// args
		var args []Value
		for _, argOp := range argOps {
//.........这里部分代码省略.........