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