FUNCTION - source-solutions/HELP GitHub Wiki

Functions can only be used as part of an expression within a statement; they may take input values between parentheses and produce a return value. For example, in PRINT ABS(-1) the ABS function is used in an expression within a PRINT statement; in Y = SQR(X) + 2 the SQR function is used in an expression within a LET statement.

For some function, including those that do not take an input, parentheses are optional. However, for maximum compatibility you may want to include them anyway.