What is Dialyzer and how does success typing differ from static typing?

Dialyzer is Erlang's static analysis tool for finding type discrepancies — but it works differently from a conventional static type checker like those in typed languages. Instead of requiring every value to be annotated and rejecting code that can't be proven correct up front, Dialyzer uses success typing: it infers the types a function could actually succeed with by analyzing the code as written, and only flags something when it can prove a call would definitely fail.

-spec add(integer(), integer()) -> integer().
add(A, B) -> A + B.

bad() -> add(1, "two").   %% Dialyzer flags this: "two" can never succeed here

Because success typing is optimistic by design — it only reports what's provably wrong, never what merely isn't proven right — it produces very few false positives, unlike stricter type systems that might reject valid-but-hard-to-prove code. -spec annotations are optional hints that sharpen Dialyzer's analysis, not required declarations the way type annotations are in a conventional statically typed language.

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