What is the difference between 'a and '_l?

Question

What is the difference between \'a and \'_l?

I was looking at this error, and couldn\'t comprehend it:

Error: This express         


        

Answer 1

There is a weak type variable in the error message from the compiler, but I am not sure that the weak variable is related to the error (I do not see how the status of the type variable could be the cause of this message).

Are you sure that you are not defining two types polynomial? That's the question just above the two pointed out by gashe in his answer in the FAQ ("Error message: a type is not compatible with itself").