my %constraint =
#    Prompter   Add to  What to print on  Use this to check that   Conversion
#      type     prompt  invalid input     input is valid           function
#    ========   ======  ================  ======================   ==========
      ::Int  => [': ', 'a valid integer', /^ \h* <integer> \h* $/, *.Int      ],
      ::Num  => [': ', 'a valid number',  /^ \h* <number>  \h* $/,   +*       ],
      ::Bool => ['? ', '"yes" or "no"',   /^ \h* <yesno>   \h* $/, {?/<yes>/} ],
    SemiBool => ['? ', '"yes" or "no"',   /^ \h* \S+       \h* $/, {?/<yes>/} ],
 CapSemiBool => ['? ', '"Yes" for yes',   /^ \h* <Yes>     \h* $/, {?/<yes>/} ],
 CapFullBool => ['? ', '"Yes" or "No"',   /^ \h* <YesNo>   \h* $/, {?/<yes>/} ],
         Any => [': ', 'anything',        /      <null>         /, { $^self } ];


    my ($punct, $description, $match, $extractor)
        = %constraint{$type} // %constraint<Any>;

# does not work in current Rakudo: the entire Array in %constraint{$type} is assigned to $punct,
# while the remainder of $description, etc get Any().