isval( NULL )
isval( NA )
isval( c( NA , NULL ) )
isval( c( 1, 2, 3 ) )
isval( c( NA, 2, 3 ) )
isval( c( 1, 2, NA ) ) # only the first values is checked, so this will come back FALSE.
isval( c( NULL, 2, 3 ) ) # NULL values get skipped in a vector.
isval( data.frame() )
isval( dplyr::group_by( dplyr::select( cars, speed, dist ), speed ) ) # test a tibble.
isval( "#VALUE!" ) # test an excel error code.
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