##Hindley-Milner type inference. Algorithm W.
##References:
- Generalizing Hindley-Milner Type Inference Algorithms by B. Heeren, J. Hage, and D. Swierstra. Technical report (2002).
- Principal type-schemes for functional programs by L. Damas and R. Milner. POPL'82 (1982).
- Hindley-Milner Type System (Wiki).
- Algorithm W Step by Step by M. Grabmuller (2006).