Reading the documentation

[Chris Angelico]

On Fri, Aug 25, 2017 at 11:25 AM, Ben Bacarisse <ben.usenet at bsb.me.uk> wrote:
> The use as a noun is not covered here, though it is only a small step
> from other places where membership of a mathematical set has turned the
> adjective into a noun.  "Rational" and "real" started out as adjectives,
> but their use as nouns is now widespread.  "The function returns a
> real".  "The result is a rational".  It's much less common for complex
> and integral, to the point that it sounds wrong to me.

This is a common thing in English (and many other languages). When you
find yourself frequently using similar phrases, you abbreviate them:

* real number -> real
* rational number -> rational
* complex number -> complex

Thus the adjective acquires a new meaning as a noun. As my mother (and
grammar teacher) drummed into me: No word is a part of speech unless
it appears in context.

ChrisA