You could discuss this. 'Fun' is a noun that people some years ago started to use as an adjective. Today, some dictionaries still do not list it as an adjective, and some people (mainly older people) still consider it substandard English.
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CliveI think that in several more years the adjective 'fun' will be part of standard English.Fun is used as a simple adjective by natives all the time, so I think it's standard by now. It's the comparative forms (especially funner, to my ear) that are not yet generally accepted.