Haskell - “How can I use ”if“ statement in ”do" block properly? [duplicate]

Answer 1

if in Haskell must always have a then and an else. So this will work:

action = do
    isdir <- doesDirectoryExist path
    if not isdir
        then handleWrong
        else return ()
    doOtherActions

Equivalently, you can use when from Control.Monad:

action = do
    isdir <- doesDirectoryExist path
    when (not isdir) handleWrong
    doOtherActions

Control.Monad also has unless:

action = do
    isdir <- doesDirectoryExist path
    unless isdir handleWrong
    doOtherActions

---

Note that when you tried

action = do
    isdir <- doesDirectoryExist path
    if(not isdir)
        then do handleWrong
        else do
    doOtherActions

it was parsed as

action = do
    isdir <- doesDirectoryExist path
    if(not isdir)
        then do handleWrong
        else do doOtherActions