Why does Haskell give me this parse error on input '->'?

I have looked through stackoverflow trying to figure this out, since I see alot of questions titled the same as mine. But I get this parse error in a Haskell file which worked perfectly the last time I opened it. I get this error-message

One defines a signature for a function by writing the name of the function, then two consecutive colons (::), and then the signature. You forgot the double colon, you thus write the signature of your concat function with:

--     ↓↓ double colon
concat :: [[a]] -> [a]