x::xs or x:xs in haskell is a very common naming pattern for pattern matching on lists. head and tail are functions that operate over lists so using them as a v

x::xs or x:xs in haskell is a very common naming pattern for pattern matching on lists. head and tail are functions that operate over lists so using them as a variable would be considered name shadowing. i is good to use as a loop counter in C for the same reason, its widely recognized and people will know what it means.