package hcl
// AbsTraversalForExpr attempts to interpret the given expression as
// an absolute traversal, or returns error diagnostic(s) if that is
// not possible for the given expression.
//
// A particular Expression implementation can support this function by
// offering a method called AsTraversal that takes no arguments and
// returns either a valid absolute traversal or nil to indicate that
// no traversal is possible. Alternatively, an implementation can support
// UnwrapExpression to delegate handling of this function to a wrapped
// Expression object.
//
// In most cases the calling application is interested in the value
// that results from an expression, but in rarer cases the application
// needs to see the the name of the variable and subsequent
// attributes/indexes itself, for example to allow users to give references
// to the variables themselves rather than to their values. An implementer
// of this function should at least support attribute and index steps.
func AbsTraversalForExpr(expr Expression) (Traversal, Diagnostics) {
type asTraversal interface {
AsTraversal() Traversal
}
physExpr := UnwrapExpressionUntil(expr, func(expr Expression) bool {
_, supported := expr.(asTraversal)
return supported
})
if asT, supported := physExpr.(asTraversal); supported {
if traversal := asT.AsTraversal(); traversal != nil {
return traversal, nil
}
}
return nil, Diagnostics{
&Diagnostic{
Severity: DiagError,
Summary: "Invalid expression",
Detail: "A single static variable reference is required: only attribute access and indexing with constant keys. No calculations, function calls, template expressions, etc are allowed here.",
Subject: expr.Range().Ptr(),
},
}
}
// RelTraversalForExpr is similar to AbsTraversalForExpr but it returns
// a relative traversal instead. Due to the nature of HCL expressions, the
// first element of the returned traversal is always a TraverseAttr, and
// then it will be followed by zero or more other expressions.
//
// Any expression accepted by AbsTraversalForExpr is also accepted by
// RelTraversalForExpr.
func RelTraversalForExpr(expr Expression) (Traversal, Diagnostics) {
traversal, diags := AbsTraversalForExpr(expr)
if len(traversal) > 0 {
ret := make(Traversal, len(traversal))
copy(ret, traversal)
root := traversal[0].(TraverseRoot)
ret[0] = TraverseAttr{
Name: root.Name,
SrcRange: root.SrcRange,
}
return ret, diags
}
return traversal, diags
}
// ExprAsKeyword attempts to interpret the given expression as a static keyword,
// returning the keyword string if possible, and the empty string if not.
//
// A static keyword, for the sake of this function, is a single identifier.
// For example, the following attribute has an expression that would produce
// the keyword "foo":
//
// example = foo
//
// This function is a variant of AbsTraversalForExpr, which uses the same
// interface on the given expression. This helper constrains the result
// further by requiring only a single root identifier.
//
// This function is intended to be used with the following idiom, to recognize
// situations where one of a fixed set of keywords is required and arbitrary
// expressions are not allowed:
//
// switch hcl.ExprAsKeyword(expr) {
// case "allow":
// // (take suitable action for keyword "allow")
// case "deny":
// // (take suitable action for keyword "deny")
// default:
// diags = append(diags, &hcl.Diagnostic{
// // ... "invalid keyword" diagnostic message ...
// })
// }
//
// The above approach will generate the same message for both the use of an
// unrecognized keyword and for not using a keyword at all, which is usually
// reasonable if the message specifies that the given value must be a keyword
// from that fixed list.
//
// Note that in the native syntax the keywords "true", "false", and "null" are
// recognized as literal values during parsing and so these reserved words
// cannot not be accepted as keywords by this function.
//
// Since interpreting an expression as a keyword bypasses usual expression
// evaluation, it should be used sparingly for situations where e.g. one of
// a fixed set of keywords is used in a structural way in a special attribute
// to affect the further processing of a block.
func ExprAsKeyword(expr Expression) string {
type asTraversal interface {
AsTraversal() Traversal
}
physExpr := UnwrapExpressionUntil(expr, func(expr Expression) bool {
_, supported := expr.(asTraversal)
return supported
})
if asT, supported := physExpr.(asTraversal); supported {
if traversal := asT.AsTraversal(); len(traversal) == 1 {
return traversal.RootName()
}
}
return ""
}
