typefilter

Typefilter is a concept in programming that refers to selecting elements from a collection based on their type. A type filter applies a type check to each element and yields or collects only those that satisfy the specified type criterion. It is commonly used when working with heterogeneous collections containing values of multiple types.

Language-specific approaches vary. In Python, type filtering often uses isinstance or type comparisons. For example, a list comprehension like [x for x in data if isinstance(x, int)] returns only integer elements. In C#, the OfType<T> extension on enumerable sequences returns elements that are of type T. In JavaScript, type checks may use typeof for primitive types or instanceof for objects created from a class constructor to filter elements by their runtime type.

Key considerations include how strict the type check is. isinstance in Python accepts subclasses, so a value of a subclass of int would pass the filter, while type(x) == int would not. Performance is typically linear in the size of the input collection, since each element must be inspected. In statically typed languages, type filtering can interact with generics and cast safety, and may be optimized by the language’s runtime or compiler.

Common uses include data cleaning of mixed-type datasets, extracting values for typed processing, and preparing streams for typed handlers or serializers. Limitations include reliance on runtime type information, potential loss of information if non-matching elements are discarded, and the need to design the type system carefully to avoid surprising results.

See also: filtering, type checking, reflection, generics.