That sounds fun.句中fun 是形容词还是不可数名词？

有人把它看作是个名词:enjoyment; a good time They had a lot of fun at the party; Isn't this fun!

有sound like fun的说法:sound like fun:to seem to be something that you would enjoy
The weekend away sounds like fun.

sound fun中的fun可以看成是抽象名词带有形容词的特性,其他的这类词还有friend等.尽管有人认为这里的fun是形容词.

fun用作形容词时,通常用作定语:
1.Informal . of or pertaining to fun, esp. to social fun: a fun thing to do; really a fun person. 2.Informal . whimsical; flamboyant: The fashions this year are definitely on the fun side.

以下是摘自美国传统词典的用法(The American Heritage® Dictionary of the English Language, Fourth Edition copyright ©2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved.):

Usage Note: The use of fun as an attributive adjective, as in a fun time, a fun place, probably originated in a playful reanalysis of the use of the word in sentences such as It is fun to ski, where fun has the syntactic function of adjectives such as amusing or enjoyable. The usage became popular in the 1950s and 1960s, though there is some evidence to suggest that it has 19th-century antecedents, but it can still raise eyebrows among traditionalists. The day may come when this usage is entirely unremarkable, but writers may want to avoid it in more formal contexts.