¶Hindley1969
Robin Hindley. “The Principal Type-Scheme of an Object in Combinatory Logic”. Transactions of the American Mathematical Society. Vol. 146 (Dec., 1969), pp. 29-60 (32 pages)
¶Notes
This is one of the famous papers that introduces Hindley-Milner type inference. Hindley's paper is focused on combinatory logic, not on the lambda calculus.