Hi folks,
(Apologies if there is duplication, I didn't see my post archived so I assumed I had to subscribe to be able to post)
In the hope of tapping the collective wisdom, I'd like to share a problem which seems to be worth sharing: how to recursively update a single record instance with a list of IO tainted functions.
A simple pure IO-less version of what I am trying to do is probably the best way to explain this:
<pure_version>
module Test
where
data Person = Person {name :: String, age :: Int} deriving Show
setName :: String -> Person -> Person
setName s p = p {name=s}
setAge :: Int -> Person -> Person
setAge i p = p {age=i}
update :: [Person -> Person] -> Person -> Person
update [] p = p
update
[f] p = f p
update (f:fs) p = update fs p'
where
p' = f p
p1 = Person {name="sue", age=12}
p2 = update [(setName "sam"), (setAge 32)] p1
</pure_version>
This works very nicely.
Now if the setter functions involve some IO, I believe the type signatures should probably look like this:
setName :: String -> Person -> IO Person
setAge :: Int -> Person -> IO Person
update :: [Person -> IO Person] -> Person -> IO Person
and the setter functions should look like this for example:
setName :: String -> Person -> IO Person
setName s p = do
putStrLn "setting name"
return p {name=s}
setAge :: Int -> Person -> IO Person
setAge i p = do
putStrLn "setting age"
return p {age=i}
but I'm stuck on how
the update function would look.. Any help would be much appreciated.
Best,
AK
