getting error for VHDL shift_left operation

Question

I have been searching this for a while and have not been able to replicate any posted solutions online so I was hoping some of you wonderful people could help me out. I am creating an ALU. i have a two 32 bit inputs and one 32 bit output along with a 5 bit shamt and a 4 bit control signal. My code is as follows with the error location commented on.

library ieee;
use ieee.std_logic_1164.all;
use IEEE.STD_LOGIC_UNSIGNED.ALL;
use ieee.numeric_std.all;

Entity mips_alu IS
    PORT(ALUControl : IN STD_LOGIC_VECTOR(3 DOWNTO 0);
    inputA, inputB      : IN STD_LOGIC_VECTOR(31 DOWNTO 0);
    shamt                   : IN STD_LOGIC_VECTOR(4 DOWNTO 0);
    Zero                    : OUT STD_LOGIC;
    ALU_Result          : OUT STD_LOGIC_VECTOR(31 DOWNTO 0)
    );
END mips_alu;

ARCHITECTURE behavior of mips_alu IS
BEGIN
    PROCESS(ALUControl)
    BEGIN
        CASE ALUControl is
        WHEN "0000" =>
            ALU_Result <= inputA AND inputB;
        WHEN "0001" =>
            ALU_Result <= inputA OR inputB;
        WHEN "0010" =>
            ALU_Result <= inputA + inputB;
        WHEN "0110" =>
            ALU_Result <= inputA - inputB;
        WHEN "0111" =>
            IF (inputA < inputB) THEN
                ALU_Result <= inputA;
            END IF;
        WHEN "1000" =>
            ALU_Result <= shift_left(inputB, to_integer(unsigned(shamt)));
            -- The line above is where i get my first error, The following lines will have the same issue
        WHEN "1001" =>
            ALU_Result <= shift_right(inputB, shamt);
        WHEN "1010" =>
            ALU_Result <= shift_left(inputB, inputA);
        WHEN "1011" =>
            ALU_Result <= shift_right(inputB, inputA);
        WHEN "1100" =>
            ALU_Result <= inputA NOR inputB;
        WHEN "1101" =>
            ALU_Result <= inputB(31 DOWNTO 16);
        WHEN OTHERS =>
            ALU_Result <= inputA;
        END CASE;
    END PROCESS;
END behavior;

The error I am getting says:

10511 VHDL Qualified Expression error at mips_alu.vhd(33): shift_left type specified in qualified expression must match std_logic_vector type that is implied for expression by context

I have tried several variations of this so if i am missing something please let me know, all help is appreciated as I am a bit of a novice to vhdl

Answer 1

In the numeric_std document you can find the function shift_left or shift_right. In both descriptions there you can see that function SHIFT_LEFT (ARG: UNSIGNED; COUNT: NATURAL) return UNSIGNED;. So you need to use casting of your std_logic_vector to unsigned.
In your case it will looks like:

ALU_Result <= std_logic_vector(shift_left(unsigned(inputB), to_integer(unsigned(shamt))));

end etc. for other lines where you use shift_left or shift_right function.