You can used the case select to do it.
For example shows below.
always @*
begin
case(a)
3'b000: s = 8'b0000_0001;
3'b001: s = 8'b0000_0010;
3'b010: s = 8'b0000_0100;
3'b011: s = 8'b0000_1000;
3'b100: s = 8'b0001_0000;
3'b101: s = 8'b0010_0000;
3'b110: s = 8'b0100_0000;
3'b111: s = 8'b1000_0000; endcase
end