performance of monads

[Manuel M. T. Chakravarty]

Eric Allen Wohlstadter <wohlstad@cs.ucdavis.edu> wrote,

> I see a lot of literature that says that monads "simulate" the effects of
> imperative programming concepts. It seems to me that the relative
> performance of monadic implementations must be equivalant to imperative
> ones to provide a strong case for functional programming. For example, in
> the Exception handling monad it seems that the "bind" function must be
> called continously to pass the Error value back to an error handling
> function. However, in imperative languages we can make an immediate
> non-local transfer of control. Similiar is the way State is handled. Do
> compilers for haskell do any sort of optimization on these monadic
> operations or is it all as ineffecient as it looks.

Monads such as the IO monad are entirely compiled away by an
optimising Haskell compiler.  So, for example, it would use
techniques similar to that of a compiler for an imperative
language to actually implement exceptions.

You can regard the monad as a functional interface to the
imperative functionality and the "simulation-based"
implementation as a specification of it's semantics, but the
generated code is free to use whatever optimisation are
possible as long as the same semantics is realised.

Cheers,
Manuel

PS: These optimisations will usually only apply to monads
    that are defined as part of the system libraries of a
    Haskell system, not to user-defined ones (unless a user
    uses non-standard system features to implement the
    monad).