57 lines
2.4 KiB
Go
57 lines
2.4 KiB
Go
// Package customdecode contains a HCL extension that allows, in certain
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// contexts, expression evaluation to be overridden by custom static analysis.
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//
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// This mechanism is only supported in certain specific contexts where
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// expressions are decoded with a specific target type in mind. For more
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// information, see the documentation on CustomExpressionDecoder.
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package customdecode
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import (
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"github.com/hashicorp/hcl/v2"
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"github.com/zclconf/go-cty/cty"
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)
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type customDecoderImpl int
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// CustomExpressionDecoder is a value intended to be used as a cty capsule
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// type ExtensionData key for capsule types whose values are to be obtained
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// by static analysis of an expression rather than normal evaluation of that
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// expression.
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//
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// When a cooperating capsule type is asked for ExtensionData with this key,
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// it must return a non-nil CustomExpressionDecoderFunc value.
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//
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// This mechanism is not universally supported; instead, it's handled in a few
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// specific places where expressions are evaluated with the intent of producing
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// a cty.Value of a type given by the calling application.
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//
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// Specifically, this currently works for type constraints given in
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// hcldec.AttrSpec and hcldec.BlockAttrsSpec, and it works for arguments to
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// function calls in the HCL native syntax. HCL extensions implemented outside
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// of the main HCL module may also implement this; consult their own
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// documentation for details.
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const CustomExpressionDecoder = customDecoderImpl(1)
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// CustomExpressionDecoderFunc is the type of value that must be returned by
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// a capsule type handling the key CustomExpressionDecoder in its ExtensionData
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// implementation.
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//
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// If no error diagnostics are returned, the result value MUST be of the
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// capsule type that the decoder function was derived from. If the returned
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// error diagnostics prevent producing a value at all, return cty.NilVal.
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type CustomExpressionDecoderFunc func(expr hcl.Expression, ctx *hcl.EvalContext) (cty.Value, hcl.Diagnostics)
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// CustomExpressionDecoderForType takes any cty type and returns its
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// custom expression decoder implementation if it has one. If it is not a
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// capsule type or it does not implement a custom expression decoder, this
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// function returns nil.
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func CustomExpressionDecoderForType(ty cty.Type) CustomExpressionDecoderFunc {
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if !ty.IsCapsuleType() {
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return nil
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}
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if fn, ok := ty.CapsuleExtensionData(CustomExpressionDecoder).(CustomExpressionDecoderFunc); ok {
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return fn
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}
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return nil
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}
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