Is it a whole number?

Haskell, 27 16 bytes

This function checks whether x is contained in the list of nonnegative integers that are not greater than x.

f x=elem x[0..x]

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APL (Dyalog), 3 bytes

⌊≡|

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Note that f← has been prepended in the TIO link due to technical limitations, but it's not normally needed.


Wolfram Language (Mathematica), 17 15 14 bytes

Saved 1 byte thanks to Not a tree!

#>=0==#~Mod~1&

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First Mathematica answer \o/

Mathematica, 15 bytes

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#==⌊[email protected]#⌋&