-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 On 9/18/10 16:30 , Gaius Hammond wrote:
Oh, but I am already in the IO monad there aren't I.... And doQuery *isn't* but I have declared that it *should be* (e.g. an instance of typeclass MonadIO). Is that accurate?
You're in ReaderT at that point. If you "lift" a monadic expression from there, it will be run in WriterT. Since IO must always be at the base(*) of any stack of monad transformers it is used in, the MonadIO class provides a convenience function liftIO that lifts its argument directly to the IO monad without having to step through any intermediates. _______ * we call it "lifting" but we also claim IO is at the base of the stack, and I at least usually expect a base to be at the bottom. Nice and confusing :) - -- brandon s. allbery [linux,solaris,freebsd,perl] allbery@kf8nh.com system administrator [openafs,heimdal,too many hats] allbery@ece.cmu.edu electrical and computer engineering, carnegie mellon university KF8NH -----BEGIN PGP SIGNATURE----- Version: GnuPG v2.0.10 (Darwin) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org/ iEYEARECAAYFAkyWopIACgkQIn7hlCsL25VRAwCfSLR1M++NqB1wPFbI8LQoD1Lk GS4Anitq5B3KDectcCF18HGDb5XqRxIo =tBJU -----END PGP SIGNATURE-----