Apply

Apply[ func, expr ]

Apply[ func, expr ] replaces the leading function of expr with func. In case expr has no leading function, func is applied directly to expr.

Example in out
The function plus applied to the list {2,3}. Apply[ plus, {2,3} ] plus[2,3]
The function times applied to the addition 2+3. Apply[ times, 2+3 ] times[2,3]
The function List applied to the variable poly defined as 2x^2 + 3x + 6 in the script to obtain a list of its terms. Apply[List, poly ] {2x^2, 3x, 6}
The function List applied to an expression without a function. Apply[List, 4] {4}
Applying a function to a term without a leading function after evaluation. Apply[sqrt, CalcPlus[2,3] ] sqrt[5]