If in do issues

[garyb]

The impossible happened in desugarDo:

test = do 
  if true 
  then return true 
  else return false

Let statement cannot be the last statement in a do block:

test = do 
  let x = 1
  if true 
  then return true 
  else return false

[paf31]

Both of these can be fixed by indenting the then and else lines:

test = do 
  if true 
    then return true 
    else return false

test = do 
  let x = 1
  if true 
    then return true 
    else return false

[michaelficarra]

I've managed to trigger the compiler crash with the same error as @garyb (using 0.6.9.5):

psc: The impossible happened in desugarDo

I believe it was code like this that triggered it:

... $ \a -> do
  if a then do
    ...
  else do
    ...

I fixed the problem by removing that first unnecessary do.

[sharkdp]

Possibly related: the same compiler error (the impossible happened..) also appears for empty do blocks. It would be convenient if the compiler would throw a compile error (Error at ...: Empty do block).