The standard library has lots of side-effecting methods which don't come with ! suffix. On the same Array type that has reject!, there's delete, pop, push, among others. Only some of them have pure equivalents - e.g. last for pop.
Push and pop don't have it because they mutate the array like they do in almost every non-hardcore-functional language, so that's the expected behavior.
Things like map! and reject! have them because there _is_ another version, and _this_ one might bite you. It has nothing to do with side-effects or immutability per se, that's just the most common case.
1) There's another, similar, method without the suffix.
2) The ! version has some side effect (usually mutating something or throwing an exception) that the non-! doesn't, which doesn't mean that the non-! has no side effects (`ApplicationRecord#save` and `#save!` both have side effects, but only `#save!` throws on failure).
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Is there any idiomatic Ruby library where ! methods don't follow this convention?