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<snip...> 
> parameters:
>   env:
>    type: string
>    default: prod
> 
>   is_prod:
>     type: boolean
>     default: {equals: {get_param, env}}
> 
> From an interface standpoint this seems much cleaner and more intuitive than
> the other solutions discussed IMHO, but I suspect it's potentially harder to
> implement.
> 
> Your original example gets much cleaner too if we allow all intrinsic function
> (except get_attr) in the scope of parameters:
> 
> parameters:
>   host:
>     type: string
>   port:
>     type: string
>   endpoint:
>     type: string
>     default:
>       str_replace:
>         template:
>            http://HOSTORT/
>         params:
>            HOST: {get_param: host}
>            PORT: {get_param: port}
> 
> Steve

+1 to restricting the variable evaluation into the 'parameters' section
which we can treat as the 'static' part of a template. Maybe that is one
of the reasons why aws has a 'conditions' section to avoid getting into
the lazy evaluation mess?

Regards,
  Qiming