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Robin Milner presented a polymorphic type discipline and its inference algorithm in 1978. Algorithm W derives types from program structure while using unification to satisfy type constraints; suitable let-bound expressions receive reusable type schemes. This work is a foundation of Hindley-Milner type inference.^[ref]

For example, an identity function has type $\alpha \to \alpha$. Generalizing a let-bound identity permits separate uses at integer and Boolean types, whereas one lambda-bound parameter has one shared type within its scope. The discipline therefore gives precise rules for reuse rather than treating every occurrence as independently polymorphic. Its type-soundness claim concerns specified runtime type errors, not every possible program failure.