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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. |
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| README.md | ||
| tryfunc_test.go | ||
| tryfunc.go | ||
"Try" and "can" functions
This Go package contains two cty functions intended for use in an
hcl.EvalContext when evaluating HCL native syntax expressions.
The first function try attempts to evaluate each of its argument expressions
in order until one produces a result without any errors.
try(non_existent_variable, 2) # returns 2
If none of the expressions succeed, the function call fails with all of the errors it encountered.
The second function can is similar except that it ignores the result of
the given expression altogether and simply returns true if the expression
produced a successful result or false if it produced errors.
Both of these are primarily intended for working with deep data structures
which might not have a dependable shape. For example, we can use try to
attempt to fetch a value from deep inside a data structure but produce a
default value if any step of the traversal fails:
result = try(foo.deep[0].lots.of["traversals"], null)
The final result to try should generally be some sort of constant value that
will always evaluate successfully.
Using these functions
Languages built on HCL can make try and can available to user code by
exporting them in the hcl.EvalContext used for expression evaluation:
ctx := &hcl.EvalContext{
Functions: map[string]function.Function{
"try": tryfunc.TryFunc,
"can": tryfunc.CanFunc,
},
}