69 lines
2.4 KiB
Go
69 lines
2.4 KiB
Go
package hcl
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type unwrapExpression interface {
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UnwrapExpression() Expression
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}
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// UnwrapExpression removes any "wrapper" expressions from the given expression,
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// to recover the representation of the physical expression given in source
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// code.
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//
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// Sometimes wrapping expressions are used to modify expression behavior, e.g.
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// in extensions that need to make some local variables available to certain
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// sub-trees of the configuration. This can make it difficult to reliably
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// type-assert on the physical AST types used by the underlying syntax.
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//
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// Unwrapping an expression may modify its behavior by stripping away any
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// additional constraints or capabilities being applied to the Value and
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// Variables methods, so this function should generally only be used prior
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// to operations that concern themselves with the static syntax of the input
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// configuration, and not with the effective value of the expression.
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//
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// Wrapper expression types must support unwrapping by implementing a method
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// called UnwrapExpression that takes no arguments and returns the embedded
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// Expression. Implementations of this method should peel away only one level
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// of wrapping, if multiple are present. This method may return nil to
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// indicate _dynamically_ that no wrapped expression is available, for
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// expression types that might only behave as wrappers in certain cases.
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func UnwrapExpression(expr Expression) Expression {
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for {
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unwrap, wrapped := expr.(unwrapExpression)
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if !wrapped {
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return expr
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}
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innerExpr := unwrap.UnwrapExpression()
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if innerExpr == nil {
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return expr
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}
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expr = innerExpr
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}
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}
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// UnwrapExpressionUntil is similar to UnwrapExpression except it gives the
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// caller an opportunity to test each level of unwrapping to see each a
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// particular expression is accepted.
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//
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// This could be used, for example, to unwrap until a particular other
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// interface is satisfied, regardless of wrap wrapping level it is satisfied
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// at.
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//
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// The given callback function must return false to continue wrapping, or
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// true to accept and return the proposed expression given. If the callback
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// function rejects even the final, physical expression then the result of
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// this function is nil.
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func UnwrapExpressionUntil(expr Expression, until func(Expression) bool) Expression {
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for {
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if until(expr) {
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return expr
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}
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unwrap, wrapped := expr.(unwrapExpression)
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if !wrapped {
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return nil
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}
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expr = unwrap.UnwrapExpression()
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if expr == nil {
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return nil
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}
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}
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}
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