simple question

[Tom Schrijvers]

Cagdas Ozgenc wrote:

> books2 :: Database2 -> Person -> [Book]
> 
> books2 db person = head [ snd tuple | tuple <- db, fst tuple == person ]
> 
> borrowers2 :: Database2 -> Book -> [Person]
> borrowers2 db book = [ person | (person, books) <- db, book <- books ]

You can't pattern match against a variable (like equal x x = True). But
that is what you seem to be doing in book <- books.
However, Haskell doesn't give you an error in this case (as it would
with my equal example) because the book in book <- books is considered a
new veriable that is hiding the other book variable.
What you wrote is the same as

> borrowers2 db book = [ person | (person, books) <- db, x <- books ]

where x <- books has no meaning (except maybe the person should have at
least one book).

Try this instead:

> borrowers2 db book = [ person | (person, books) <- db, elem book books]

As for books2, use (person, books) <- db instead of tuple. Looks better
than fst and snd in my opinion.

Tom