@Adrian There is a datatype

Init

that answeres if a value is a viable initialization of a data type: See the example below:

henrik$ puppet apply -e 'notice "1.2.3" =~ Init[SemVer]'
Notice: Scope(Class[main]): true

henrik$ puppet apply -e 'notice "present" =~ Init[SemVer]'
Notice: Scope(Class[main]): false