Parameter validation using fail

[ReubenFrankel]

is it possible to perform simple parameter validation with the new fail function without some kind of assignment (i.e an empty expression)?

This works

#disable-next-line no-unused-vars
var _ = empty(param1) && empty(param2) && fail('paramA or paramB must be provided')

but these

empty(param1) && empty(param2) && fail('paramA or paramB must be provided')

(empty(param1) && empty(param2) && fail('paramA or paramB must be provided'))

result in compilation errors.

I was looking to see if docs covered this but can't find any on fail: https://learn.microsoft.com/en-us/azure/azure-resource-manager/bicep/operators-logical: https://learn.microsoft.com/en-us/azure/azure-resource-manager/bicep/bicep-functions

[ReubenFrankel]

Additionally, I cannot define multiple validation expressions that assign to the same stub var - I would have to keep declaring new identifiers (i.e. var _ = ..., var __ = ...). Neither of these examples compile:

#disable-next-line no-unused-vars
var _ = empty(param1) && empty(param2) && fail('paramA or paramB must be provided')
#disable-next-line no-unused-vars
var _ = !empty(param1) && !empty(param2) && fail('Only one of paramA and paramB should be provided')

#disable-next-line no-unused-vars
var _ = empty(param1) && empty(param2) && fail('paramA or paramB must be provided')
_ = !empty(param1) && !empty(param2) && fail('Only one of paramA and paramB should be provided')

[jeskew]

All values in Bicep are immutable, so reassignments are blocked. You can use an array if you need a series of assertions; the following will compile:

var __assertions = [
  empty(param1) && empty(param2) && fail('paramA or paramB must be provided')
  !empty(param1) && !empty(param2) && fail('Only one of paramA and paramB should be provided')
]

The language doesn't really have standalone statements, so the expressions do need to be part of a declaration of some kind.

[ReubenFrankel]

Fair enough. I figured this

so reassignments are blocked

was the case anyway, but included it for demonstration purposes.

This looks OK:

var __assertions = [
 empty(param1) && empty(param2) && fail('paramA or paramB must be provided')
 !empty(param1) && !empty(param2) && fail('Only one of paramA and paramB should be provided')
]]

I guess I could group assertions together and "label" them with the var name. I do wonder if there is a best-practice I should be following here instead though - like if only one of paramA and paramB is required, they should exists as the same param; or use of fail as close as possible to paramA/paramB application?

[jeskew]

You could use a tagged union:

type aType = {
  kind: 'a'
  value: ...
}

type bType = {
  kind: 'b'
  value: ...
}

@discriminator('kind')
param aOrB aType | bType

It's a little more verbose and would be an observable change to whatever is consuming the template, but it offloads enforcement of the must-be-one-or-the-other semantic to the ARM type checker and has better intellisense in Bicep.

[ReubenFrankel]

Thanks, I'll take a look into that. 🙂