Question

Sometimes I see code with the following expression:

example = example' []

Or what is the different between these functions?

foldl
foldl'

Answer 1

' is simply another identifier character like any other. foldl' is a completely separate function from foldl; it could just as well be called strictFold. It’s called that because it’s closely related to foldl: it’s a foldl where the accumulator is evaluated at every step, so that large thunks don’t build up. For instance, foldl (+) 0 will overflow the stack on a large list, but foldl' (+) 0 won’t.

In general, the suffixing of a ' means one of three things: foo' is either a helper definition made for the purpose of defining foo, a modified version of foo (that is, state, state', and state'' could be an initial state and two updated versions), or a strict version of foo.