On Jan 1, 2007, at 6:02 , Yitzchak Gale wrote:
Prelude Control.Monad.ST> (runST . return) 42
<interactive>:1:9: Couldn't match expected type `forall s. ST s a' against inferred type `m a1'
I think the problem is that technically runST is a data constructor (possibly not relevant) which takes a function as a parameter (definitely relevant). In the normal compositional model, (f . g) x = f (g x), you're conceptually invoking f on the result of g x (g is independent of f); here, you're lifting the function g x into the ST s a monad via f (g is dependent on f). -- 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