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Practical Foundations for Programming Languages by Robert Harper

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6 Type Safety

Most contemporary programming languages are safe (or type safe, or strongly typed). Informally, this means that certain kinds of mismatches cannot arise during execution. For example, type safety for image{num str} states that it will never arise that a number is to be added to a string, or that two numbers are to be concatenated, neither of which is meaningful.

In general type safety expresses the coherence between the statics and the dynamics. The statics may be seen as predicting that the value of an expression will have a certain form so that the dynamics of that expression is well-defined. Consequently, evaluation cannot “get ...

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