Pattern matching equivalent variables in Haskell, like in Prolog

Question

In prolog, we can do something like the following:

myFunction a (a:xs) = ...

This is, when the 1st argument of myFunction is the same as the first item of the list that's in the 2nd argument, this function will evaluate to ....

My question now is... how to accomplish a similar thing in Haskell? I have the idea that Prolog's Pattern Matching is more expressive than Haskell's. I've been trying to code that in Haskell and I'm having trouble -- either I am using invalid syntax or the above trick will simply not do.

Answer 1

Haskell doesn't do this kind of "variable matching". You'll have to explicitly put a guard on:

myFunction a (x:xs)
    | x == a = ...