Make variable treated as command

I want to create a variable in my Makefile. It will be compared in ifeq-statement. But my variable name is treated like command:

make: dif: Command not found

I have:

dif := $(shell diff file1 <(./myprog < file2))

I've been reading some manuals and testing, but nothing worked. My non-working effects of it are as seen as above.