Consider
let x = Cd ... forkIO $ ( do something with x } -- (1) print x -- (2)
How can ghc know when running line (2) that (1) hasen't changed the record? I see two solutions: a) give the forked process a copy (Then my design will collapse) but this is expensive to copy data without knowing you ned to b) use pointers and replace x ony on updating. Thus if (1) changes the title a new struct wil be created poiting to the old list but a new title String. line (2) doesn't have to care at all.
GHC knows that because in Haskell isn't possible to "update" x. x is not a variable, it's a binding. To put it simply: with IORefs (and STRefs, MVars, ...) you have references to values that you can change (inside their respective monads), much like variables, but data declarations are values, not references to values (even if GHC stores them as pointers you cannot treat them as such), so you cannot update it. So, in your example, you have more or less a relation (CD) where all the "columns" are part of primary key (and so they are not mutable). Salvatore