Some conditionals:

Conditional assignment

• good match for a circuit a circuit that needs to give preferential treatment for certain conditions or to prioritize the operations
• E.g., priority encoder
• Can handle complicated conditions. e.g.,

signal_name <= value_expr_1 when boolean_expr_1 else
               value_expr_2 when boolean_expr_2 else 
               value_expr_3 when boolean_expr_3 else
               . . .
               value_expr_n

Selected signal assignment statement (switch case)

• good match for a circuit described by a functional table
• E.g., binary decoder, multiplexer
• Less effective when an input pattern is given a preferential treatment

with select_expression select 
    signal_name <= value_expr_1 when choice_1, 
                   value_expr_2 when choice_2, 
                   value_expr_3 when choice_3, 
                   . . . value_expr_n when choice_n;