One and the same thing can be used for many purposes. A chair might be used to support a sitting person but also as a ladder to stand on while changing a light bulb or as a clothes valet. Beside having particular material qualities, which make a chair usable as such, it also has the particular designation for its use. When no chair is at hand, other things might be used as a seat, and so the designation of a thing can be changed as fast as one can turn an empty bottle crate upside down to change its purpose from a container to that of a support.

—Wikipedia intro to Hindley-Milner Type System