[Haskell-cafe] Problem with monadic formlets
Colin Paul Adams
colin at colina.demon.co.uk
Thu Aug 27 11:09:18 EDT 2009
I'm trying to validate user input against a database (using HaskellDB,
but that doesn't seem to be the problem, as replacing the database
monadic code with return True gives the same problem.
This is part of my code:
register :: Database -> XForm Registration
--register db = Registration <$> pure_user <*> passConfirmed
register db = Registration <$> (user db) <*> passConfirmed
user :: Database -> XForm String
user db = pure_user `F.checkM` F.ensureM valid error where
valid name = do
let q = do
t <- table user_table
restrict (t!user_name .==. constant name)
rs <- query db q
return $ null rs
error = "Username already exists in the database!"
pure_user :: XForm String
pure_user = input `F.check` F.ensure valid error where
input = "Username" `label` F.input Nothing
valid = (>= 3) . length
error = "Username must be three characters or longer."
passConfirmed :: XForm String
passConfirmed = fst <$> passwords `F.check` F.ensure equal error where
passwords = (,) <$> pass "Password" <*> pass "Password (confirm)"
equal (a, b) = a == b
error = "The entered passwords do not match!"
pass :: String -> XForm String
pass caption = input `F.check` F.ensure valid error where
input = caption `label` F.password Nothing
valid = (>=6) . length
error = "Password must be six characters or longer."
If I uncomment the commented line, and comment out the line after it
(in register), then everything works as expected. However, using it as
it is, one of the calls to pass gets the user's name for validation
(and consequently either fails if the user name is only 5 characters,
or the comparison of the two passwords fail (unless I type the user
name as the password).
I thought the applicative style meant the effects did not influence
one another, but here there is clear contamination. What am i doing wrong?
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