Exception: Failure "bye".
-->
-<!--
Here's an example wrapping Option around List, and vice versa:
# module LO = Monad.List.T(Monad.Option);;
- # module OL = Monad.Option.T(Monad.List);;
- # let plus = ???;;
- # OL.(run (plus mzero mid 20 >>= fun i -> mid (i+10)));;
+ # module OL = Monad.Option.P(Monad.List);; (* we use the `.P` transformer to hoist List's ++ operation *)
+ # OL.(run (mzero ++ mid 20 >>= fun x -> mid (x+10) ));;
- : int OL.result = [Some 30]
When List is on the inside, the failed results just got dropped and the computation proceeds without them.
- # LO.(run ((++) (hoist Monad.Option.mzero) (mid 20) >>= fun i -> mid (i+10)));;
+ # LO.(run (hoist Monad.Option.mzero ++ mid 20 >>= fun x -> mid (x+10) ));;
- : int LO.result = None
-On the other hand, when Option is on the inside, as in LO, failures (which we represent by `mzero`s from the Option monad, here `hoist Monad.Option.mzero`, not the List monad's own `mzero`) abort the whole computation.
+On the other hand, when Option is on the inside, as in LO, failures (which we again represent by `mzero`s from the Option monad, not the List monad's own `mzero`; but here since it's the inner monad we need to `hoist Monad.Option.mzero`) abort the whole computation. (If you instead used the List monad's `mzero`, it'd be ignored by `++` and you'd end with just `Some [30]`.)
-This is fun. Notice the difference it makes whether the second `++` is native to the outer `Monad.List`, or whether it's the inner `Monad.List`'s `++` hoisted into the outer wrapper:
+This is fun. Here is a List around a List. Notice the difference it makes whether the second `++` is native to the outer `Monad.List`, or whether it's the inner `Monad.List`'s `++` hoisted into the outer wrapper:
# module LL = Monad.List.T(Monad.List);;
- # LL.(run((++) (mid 1) (mid 2) >>= fun i -> (++) (mid i) (mid (10*i)) ));;
+ # LL.(run(mid 1 ++ mid 2 >>= fun x -> mid x ++ mid (10*x) ));;
- : int LL.result = \[[1; 10; 2; 20]]
- # LL.(run((++) (mid 1) (mid 2) >>= fun i -> hoist L.((++) (mid i) (mid (10*i)) )));;
+ # LL.(run(mid 1 ++ mid 2 >>= fun x -> hoist Monad.List.(mid x ++ mid (10*x)) ));;
- : int LL.result = [[1; 2]; [1; 20]; [10; 2]; [10; 20]]
--->
<!--
# EL.(run((++) (throw "bye") (mid 20) >>= fun i -> mid (i+10)));;