[Haskell-beginners] if True than let...

[Bernhard Lehnert]

Hi,

I'm sorry because I am absolutely sure, this is bloody obvious to the
knowing. Being a total beginner I'm stuck. In the main = do part I
wrote:

1:     if a == True then putStrLn "Yes!" else putStrLn "No."
2:     if a == True then let b = "+" else let b = "-"

Line #1 works perfectly well. 
Read line #2 as pseudocode and you'll see what I want to do. Read it in
ghci and it produces 
     "  parse error on input `='    "

I tried 'case of' but it doesn't work either. 

What am I doing wrong?
Thank you for any help,

Bernhard