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Practical Foundations for Programming Languages by Robert Harper

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8 Function Definitions and Values

In the language image{num str} we may perform calculations such as the doubling of a given expression, but we cannot express doubling as a concept in itself. To capture the general pattern of doubling, we abstract away from the particular number being doubled by using a variable to stand for a fixed, but unspecified, number to express the doubling of an arbitrary number. Any particular instance of doubling may then be obtained by substituting a numeric expression for that variable. In general an expression may involve many distinct variables, necessitating that we specify which of several possible variables is varying ...

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