On Sun, Jun 2, 2013 at 7:22 PM, Ting Lei <tin...@gmail.com> wrote: > In particular, I wanted to avoid having an IO in the return type because > introducing the impurity > (by that I mean the IO monad) for this simple task is logically > unnecessary. All examples involing >
Anything that comes into or goes out of a Haskell program is in IO, period. If you have an FFI function which is guaranteed to not change anything but its parameters and those only in a pure way, then you can use unsafeLocalState to "hide" the IO; but claiming that when it's not true can lead to problems ranging from incorrect results to core dumps, so don't try to lie about it. > a C string I have seen so far involve returning an IO something or Ptr > which cannot be converted back to a pure String. > Haskell String-s are *not* C strings. Not even slightly. C cannot work with Haskell's String type directly at all. Some kind of marshaling is absolutely necessary; there are functions in Foreign.Marshal.String that will marshal Haskell String-s to and from C strings. (String is a linked list of Char, which is also not a C char; it is a constructor and a machine word large enough to hold a Unicode codepoint. And because Haskell is non-strict, any part of that linked list can be an unevaluated thunk which requires forcing the evaluation of arbitrary Haskell code elsewhere to "reify" the value; this obviously cannot be done in the middle of random C code, so it must be done during marshaling.) -- brandon s allbery kf8nh sine nomine associates allber...@gmail.com ballb...@sinenomine.net unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net
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