Merge pull request #524 from hashicorp/f-json-is-json

json: Functions for determining if an expression or body is a JSON one

json/is.go

@@ -0,0 +1,54 @@
+package json
+
+import (
+	"github.com/hashicorp/hcl/v2"
+)
+
+// IsJSONExpression returns true if and only if the given expression is one
+// that originated in a JSON document.
+//
+// Applications aiming to be syntax-agnostic should not use this function and
+// should instead use the normal expression evaluation or static analysis
+// APIs.
+//
+// However, JSON expressions do have a unique behavior whereby they interpret
+// the source JSON differently depending on the hcl.EvalContext value passed
+// to the Value method -- in particular, a nil hcl.EvalContext returns
+// literal strings rather than interpreting them as HCL template syntax --
+// and so in exceptional cases an application may wish to rely on that behavior
+// in situations where it specifically knows the expression originated in JSON,
+// in case it needs to do some non-standard handling of the expression in that
+// case.
+//
+// Caution: The normal HCL API allows for HCL expression implementations that
+// wrap other HCL expression implementations. This function will return false
+// if given an expression of some other type that encapsulates a JSON
+// expression, even if the wrapper implementation would in principle preserve
+// the special evaluation behavior of the wrapped expression.
+func IsJSONExpression(maybeJSONExpr hcl.Expression) bool {
+	_, ok := maybeJSONExpr.(*expression)
+	return ok
+}
+
+// IsJSONBody returns true if and only if the given body is one that originated
+// in a JSON document.
+//
+// Applications aiming to be syntax-agnostic should not use this function and
+// should instead use the normal schema-driven or "just attributes' decoding
+// APIs.
+//
+// Howeer, JSON expressions do have a unique behavior whereby various different
+// source JSON shapes can be interpreted in different ways depending on the
+// given schema, and so in exceptional cases an application may need to
+// perform some deeper analysis first in order to distinguish variants of
+// different physical structure.
+//
+// Caution: The normal HCL API allows for HCL body implementations that wrap
+// other HCL body implementations. This function will return false if given an
+// expression of some other type that encapsulates a JSON body, even if
+// the wrapper implementation would in principle preserve the special
+// decoding behavior of the wrapped body.
+func IsJSONBody(maybeJSONBody hcl.Body) bool {
+	_, ok := maybeJSONBody.(*body)
+	return ok
+}