Haskell-style state transformers.
State transformers generalise state monads. They attach onto an existing monad, and allow the stateful computations to return and handle values inside that monad. For example, transforming On_error provides stateful, potentially-failing computations.
We provide two signatures for state transformers: one corresponding to the situation where the state type is fixed at the module level (S), and one leaving the state type as part of the monad type (S2). Both have corresponding make functors.
We also provide a functor To_S for fixing the state type in an arity-2 monad after the fact.
State_transform_intf contains the signatures for State_transform.
include module type of State_transform_intfmodule type Generic_types = sig ... endGeneric_types contains generic versions of the types used in Generic_builders and Generic_runners.
module type Generic_builders = sig ... endGeneric_builders contains generic versions of the 'builder' functions common to all state monad signatures.
module type Generic_runners = sig ... endGeneric_runners contains generic versions of the 'runner' functions common to all state monad signatures.
module type Fix = sig ... endFix contains the signature for fixpoint builders.
module type Generic = sig ... endGeneric contains the signature bits common to all state transformers.
module type S = sig ... endS is the signature of state monad transformers with a fixed state type.
module type Basic = sig ... endBasic is the signature that must be implemented by state systems being lifted into S_transform instances.
module type S2 = sig ... endS2 is the signature of state transformers parametrised over both value and state types.
module To_S
(M : S2)
(B : Base.T) :
S
with type state = B.t
and type 'a t = ('a, B.t) M.t
and module Inner = M.InnerTo_S flattens a S2 into an S by fixing the state type to B.t.