package travesty

  1. Overview
  2. Docs
Module type
Class type

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_intf
module type Generic_types = sig ... end

Generic_types contains generic versions of the types used in Generic_builders and Generic_runners.

module type Generic_builders = sig ... end

Generic_builders contains generic versions of the 'builder' functions common to all state monad signatures.

module type Generic_runners = sig ... end

Generic_runners contains generic versions of the 'runner' functions common to all state monad signatures.

module type Fix = sig ... end

Fix contains the signature for fixpoint builders.

module type Generic = sig ... end

Generic contains the signature bits common to all state transformers.

module type S = sig ... end

S is the signature of state monad transformers with a fixed state type.

module type Basic = sig ... end

Basic is the signature that must be implemented by state systems being lifted into S_transform instances.

module type S2 = sig ... end

S2 is the signature of state transformers parametrised over both value and state types.

Manipulating state transformers

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.Inner

To_S flattens a S2 into an S by fixing the state type to B.t.

Functors for making state transformers

module Make (B : Basic) : S with type state = B.t and module Inner = B.Inner

Make makes an S (state transformer with fixed state type) from a Basic.

module Make2 (M : Base.Monad.S) : S2 with module Inner = M

Make2 makes an S2 (state transformer with variable state type) from a Monad.S.