Type safety
property of a type system that prevents certain erroneous or undesirable program behaviours
In computer science, type safety is the extent to which a programming language discourages or prevents type errors. Type-safe languages are sometimes also called strongly or strictly typed. The behaviors classified as type errors by a given programming language are usually those that result from attempts to perform operations on values that are not of the appropriate data type, e.g. trying to add a string to an integer.
Type enforcement can be static (catching potential errors at compile time), dynamic (associating type information with values at run-time and consulting them as needed to detect imminent errors), or a combination of both. Dynamic type enforcement can often run programs that would be invalid under static enforcement, but at the cost of introducing errors at runtime.
In the context of static (compile-time) type systems, type safety usually involves (among other things) a guarantee that the eventual value of any expression will be a legitimate member of that expression's static type. The precise requirement is more subtle than this — see, for example, subtyping and polymorphism for complications.
Definitions
Intuitively, type soundness is captured by Robin Milner's pithy statement that
Well-typed programs cannot "go wrong".
In other words, if a type system is sound, then expressions accepted by that type system must evaluate to a value of the appropriate type (rather than produce a value of some other, unrelated type or crash with a type error).
Begin with the source’s own compact description: “Type safety” is property of a type system that prevents certain erroneous or undesirable program behaviours. The dossier treats that line as a proposition to test through Type, safety and property, not as a finished interpretation.
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The phrase “property of a type system that prevents certain erroneous or undesirable program behaviours” supplies a clear boundary for inquiry. It also exposes the unanswered questions: who defined that boundary, when it became stable and which sources sit outside it.
The citation trail is more important than the brevity of the summary: it shows where individual claims can be examined in context. The source revision retrieved here is dated Sep 10, 2026. The linked authority identifier is Q736866. None of the 0 selected statements returned an explicit reference.
Overview language is designed for orientation and should not be treated as a substitute for the evidence cited beneath it. The lead is largely declarative, so disagreement and counter-evidence require a deliberate search beyond the opening account. Authority statements aid reconciliation but still require their own references, qualifiers and ranks to be checked.
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This entry incorporates text from “Type safety” on English Wikipedia. Contributors are listed in the page history. Text is available under the Creative Commons Attribution-ShareAlike 4.0 License. Selected authority identifiers and statements are retrieved from Wikidata under CC0; their references and qualifiers remain part of the verification path.