How do `do` and `where` mix?

Question

From some book I have the following code snippets

mutableUpdateIO :: Int -> IO (MV.MVector RealWorld Int)
mutableUpdateIO n = do
  mvec <- GM.new (n + 1)
  go n mvec
  where
    go 0 v = return v
    go n v = (MV.write v n 0) >> go (n - 1) v

mutableUpdateST :: Int -> V.Vector Int
mutableUpdateST n =
  runST $ do
    mvec <- GM.new (n + 1)
    go n mvec
  where
    go 0 v = V.freeze v
    go n v = (MV.write v n 0) >> go (n - 1) v

like hindent indents them. Now I want to introduce all braces and semicolons, so the whitespace isn't relevant any more. Just because I am curious.

The second example suggests, that the where belongs to the whole runST $ do ... expression, but the first example suggests, that the where is somehow a part of the go n mvec statement. Reading in Haskell Report Chapter 2.7 I tried to introduce braces and semicolons in the first example like

mutableUpdateIO :: Int -> IO (MV.MVector RealWorld Int)
mutableUpdateIO n = do {
  mvec <- GM.new (n + 1);
  go n mvec;
  where {
    go 0 v = return v;
    go n v = (MV.write v n 0) >> go (n - 1) v;
  } ; }

But I get a parsing error. Why is that?

Why is the layout hindent produces for the first example mutableUpdateIO valid Haskell? Shouldn't the braces and semicolons be introduced like in my above try?

How do `do` and `where` mix?

1 Answers