Question

I’m fairly new to Haskell and I don’t fully understand this error, when I load the file hugs prints out the following “Syntax error in declaration (unexpected `;’, possibly due to bad layout)” at the line “check s1 s2 ((x,y):xs)”. I find this confusing as there isn’t a “;” in the code. If someone could explain why this is happening and how I can fix it I would be very grateful. Bellow is my code.

type Owned = String  
type Owner = String  
type Fact = (Owned,Owner)

database = [(String, String)]  
database = [("c4","c5"),("c1","c2"), ("c2", "c3"), ("c3","c4")]

owns :: Owner -> Owned -> Bool

owns s1 s2  
      | check s1 s2 database = true  
      | otherwise false

check s1 s2 ((x,y):xs)  
     | s1==x && y==s2 = true  
     | s1==x && y==s2 = (check y s2 database)  
     | otherwise false

Answer 1

Actually, there are ; in the transformed source code. The Haskell Report contains a detailed explanation about how the source code is transformed with the layout rules. One should read them once, it’s quite intuitive.

Though such errors are indeed confusing for a newcomer, the following rules of thumb apply:

• If the compiler complains about ‘;’ you have a syntax error in the last nonempty line before the line reported.
• More often than not, the error comes about due to layout (as indeed the compiler suggested). Nevertheless, there are other cases as your example shows: the compiler desperately looks for a ‘=’ in the second guard of function owns and when he finds the semicolon just before check he knows there is something wrong.