Haploids
can be naturally occurring processes like Parthenogenesis, or three processes including:

Special Thanks to Daryoush Fekri and Pioneer Hi-Bred Canada, Jim Hoare, Laima Kott, Ken Kasha, and Peter Pauls
Nitzsche, W. & G. Wenzel. (1977). Haploids in Plant Breeding. Verlag Paul Parey. Berlin and Hamburg.
Pauls, P. (1996). Course Notes for Biotechnology I.
Poehlman, J. (1979). Breeding Field Crops 2nd Ed. AVI Publishing Company, Inc. Westport Connecticut