Question

foldr (+) 0 [1,2] returns 3

How could I write it using 'where'?

foldr f 0 [1,2] where f = (+) returns "parse error on input 'where'"

Edit:
actually I’m trying to make cartesian product as in the example

cprod = foldr f [[ ]]
where f xs yss = foldr g [ ] xs
where g x zss = foldr h zss yss
where h ys uss = (x : ys) : uss

but, this again gives parse error on input where, when I’m trying to load the file.

Answer 1

... where ... is not an expression. where is associated with bindings, not expressions.

You could use let: let f = (+) in foldr f 0 [1,2], or you could use it with some binding: x = foldr f 0 [1,2] where f = (+).

---

The following is a syntactically valid version of your edited code (it’s still broken, but that’s not a syntactic issue anymore 🙂 ). You only want to use one where per binding, and you want the where to be more indented than the body of the function.

cprod = foldr f [[ ]]
  where
    f xs yss = foldr g [ ] xs
    g x zss = foldr h zss yss
    h ys uss = (x : ys) : uss

---

Looking again, I see that I misunderstood your code — you had three wheres because you wanted each one to apply to the previous functions. In that case you have to indent the wheres the way I said, e.g.

cprod = foldr f [[ ]]
  where f xs yss = foldr g [ ] xs
          where g x zss = foldr h zss yss
                  where h ys uss = (x : ys) : uss

If you insist on using where like this and don’t like deeply indented code, you can use explicit {}/; rather than layout. An extreme case would be

{
cprod = foldr f [[ ]]
where { f xs yss = foldr g [ ] xs
where { g x zss = foldr h zss yss
where { h ys uss = (x : ys) : uss } } }
}

But this isn’t usually considered good style.