Most of the time, the standard expression decoding built in to HCL is
sufficient. Sometimes though, it's useful to be able to customize the
decoding of certain arguments where the application intends to use them
in a very specific way, such as in static analysis.
This extension is an approximate analog of gohcl's support for decoding
into an hcl.Expression, allowing hcldec-based applications and
applications with custom functions to similarly capture and manipulate
the physical expressions used in arguments, rather than their values.
This includes one example use-case: the typeexpr extension now includes
a cty.Function called ConvertFunc that takes a type expression as its
second argument. A type expression is not evaluatable in the usual sense,
but thanks to cty capsule types we _can_ produce a cty.Value from one
and then make use of it inside the function implementation, without
exposing this custom type to the broader language:
convert(["foo"], set(string))
This mechanism is intentionally restricted only to "argument-like"
locations where there is a specific type we are attempting to decode into.
For now, that's hcldec AttrSpec/BlockAttrsSpec -- analogous to gohcl
decoding into hcl.Expression -- and in arguments to functions.
This uses the expression static analysis features to interpret
a combination of static calls and static traversals as the description
of a type.
This is intended for situations where applications need to accept type
information from their end-users, providing a concise syntax for doing
so.
Since this is implemented using static analysis, the type vocabulary is
constrained only to keywords representing primitive types and type
construction functions for complex types. No other expression elements
are allowed.
A separate function is provided for parsing type constraints, which allows
the additonal keyword "any" to represent the dynamic pseudo-type.
Finally, a helper function is provided to convert a type back into a
string representation resembling the original input, as an aid to
applications that need to produce error messages relating to user-entered
types.
Martin Atkins