What indentation is required for a case statement within a let statement?

Question

Working in haskell, found odd behavior, stripped it down to bare bones

This Works

a :: Bool
a = case True of
    True -> True
    False -> False

But when I try

b :: IO Bool
b = do
    let b' = case True of
        True -> True
        False -> False
    return b'

I get

ghci>:l test.hs
[1 of 1] Compiling Main             ( test.hs, interpreted )

test.hs:16:14: parse error on input ‘->’
Failed, modules loaded: none.

So I try

c :: IO Bool
c = do
    let c' = case True of
            True -> True
            False -> False
    return c'

And this works.

What? Why? Why do I need an extra indent in this case? I can't find anything on this, probably because these keyword are so short and common in everyday language. Is there some spec that explains this behavior?

Answer 1

I don't have the exact wording from the spec, but this Wikibook page explains the issue quite clearly.

The reason why it works like this is simple: to support binding multiple variables via a single let-group, such as:

c = do
    let c' = …
        d  = …
        e  = …
    return c'

Your True -> … and False -> … are mistakenly interpreted as additional variables to be bound.