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Custom expression decoding in function calls and hcldec specs #330

Merged
merged 3 commits into from
Dec 17, 2019
Merged

Custom expression decoding in function calls and hcldec specs #330

merged 3 commits into from
Dec 17, 2019

Commits on Dec 14, 2019

  1. go get github.com/zclconf/go-cty@v1.2.0 

    This includes a new feature to allow extension properties associated with
capsule types, which HCL can in turn use to allow certain capsule types
to opt in to special handing at the HCL layer.

(There are no such hooks in use as of this commit, however.)
    @apparentlymart
    apparentlymart committed Dec 14, 2019
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  2. ext/customdecode: Custom expression decoding extension 

    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.
    @apparentlymart
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  3. ext/tryfunc: Extension functions for error handling 

    The try(...) and can(...) functions are intended to make it more
convenient to work with deep data structures of unknown shape, by allowing
a caller to concisely try a complex traversal operation against a value
without having to guard against each possible failure mode individually.

These rely on the customdecode extension to get access to their argument
expressions directly, rather than only the results of evaluating those
expressions. The expressions can then be evaluated in a controlled manner
so that any resulting errors can be recognized and suppressed as
appropriate.
    @apparentlymart
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