## injection

<*mathematics*> A function, f : A -> B, is injective or
one-one, or is an injection, if and only if

for all a,b in A, f(a) = f(b) => a = b.I.e. no two different inputs give the same output (contrast many-to-one). This is sometimes called an embedding. Only injective functions have left inverses f' where f'(f(x)) = x, since if f were not an injection, there would be elements of B for which the value of f' was not unique. If an injective function is also a surjection then is it a bijection.

<*reduction*> An injection function is one which takes
objects of type T and returns objects of type C(T) where C is
some type constructor. An example is

f x = (x, 0).The opposite of an injection function is a projection function which extracts a component of a constructed object, e.g.

fst (x,y) = x.We say that f injects its argument into the data type and fst projects it out.

Last updated: 1995-03-14

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