Useful links
Click to expand
Papers:
Towards Typed Prolog - See links under SWI-Prolog for related source code.
SWI-Prolog:
library(errors)
package type_check - Link to code is dead.
However for Pack type_check – prolog/type_check.pl clicking 
links to code
package typedef
Mercury:
The Mercury Language Reference Manual - Chapter 3 - Types
Discriminated unions