author haftmann Wed Jul 14 12:27:44 2010 +0200 (2010-07-14) changeset 37817 71e5546b1965 parent 37791 0d6b64060543 child 37932 d00a3f47b607 permissions -rw-r--r--
tuned infix syntax
     1 (*  Title:      HOL/Library/State_Monad.thy

     2     Author:     Florian Haftmann, TU Muenchen

     3 *)

     4

     5 header {* Combinator syntax for generic, open state monads (single threaded monads) *}

     6

     7 theory State_Monad

     8 imports Monad_Syntax

     9 begin

    10

    11 subsection {* Motivation *}

    12

    13 text {*

    14   The logic HOL has no notion of constructor classes, so

    15   it is not possible to model monads the Haskell way

    16   in full genericity in Isabelle/HOL.

    17

    18   However, this theory provides substantial support for

    19   a very common class of monads: \emph{state monads}

    20   (or \emph{single-threaded monads}, since a state

    21   is transformed single-threaded).

    22

    23   To enter from the Haskell world,

    24   \url{http://www.engr.mun.ca/~theo/Misc/haskell_and_monads.htm}

    25   makes a good motivating start.  Here we just sketch briefly

    26   how those monads enter the game of Isabelle/HOL.

    27 *}

    28

    29 subsection {* State transformations and combinators *}

    30

    31 text {*

    32   We classify functions operating on states into two categories:

    33

    34   \begin{description}

    35     \item[transformations]

    36       with type signature @{text "\<sigma> \<Rightarrow> \<sigma>'"},

    37       transforming a state.

    38     \item[yielding'' transformations]

    39       with type signature @{text "\<sigma> \<Rightarrow> \<alpha> \<times> \<sigma>'"},

    40       yielding'' a side result while transforming a state.

    41     \item[queries]

    42       with type signature @{text "\<sigma> \<Rightarrow> \<alpha>"},

    43       computing a result dependent on a state.

    44   \end{description}

    45

    46   By convention we write @{text "\<sigma>"} for types representing states

    47   and @{text "\<alpha>"}, @{text "\<beta>"}, @{text "\<gamma>"}, @{text "\<dots>"}

    48   for types representing side results.  Type changes due

    49   to transformations are not excluded in our scenario.

    50

    51   We aim to assert that values of any state type @{text "\<sigma>"}

    52   are used in a single-threaded way: after application

    53   of a transformation on a value of type @{text "\<sigma>"}, the

    54   former value should not be used again.  To achieve this,

    55   we use a set of monad combinators:

    56 *}

    57

    58 notation fcomp (infixl "\<circ>>" 60)

    59 notation scomp (infixl "\<circ>\<rightarrow>" 60)

    60

    61 abbreviation (input)

    62   "return \<equiv> Pair"

    63

    64 text {*

    65   Given two transformations @{term f} and @{term g}, they

    66   may be directly composed using the @{term "op \<circ>>"} combinator,

    67   forming a forward composition: @{prop "(f \<circ>> g) s = f (g s)"}.

    68

    69   After any yielding transformation, we bind the side result

    70   immediately using a lambda abstraction.  This

    71   is the purpose of the @{term "op \<circ>\<rightarrow>"} combinator:

    72   @{prop "(f \<circ>\<rightarrow> (\<lambda>x. g)) s = (let (x, s') = f s in g s')"}.

    73

    74   For queries, the existing @{term "Let"} is appropriate.

    75

    76   Naturally, a computation may yield a side result by pairing

    77   it to the state from the left;  we introduce the

    78   suggestive abbreviation @{term return} for this purpose.

    79

    80   The most crucial distinction to Haskell is that we do

    81   not need to introduce distinguished type constructors

    82   for different kinds of state.  This has two consequences:

    83   \begin{itemize}

    84     \item The monad model does not state anything about

    85        the kind of state; the model for the state is

    86        completely orthogonal and may be

    87        specified completely independently.

    88     \item There is no distinguished type constructor

    89        encapsulating away the state transformation, i.e.~transformations

    90        may be applied directly without using any lifting

    91        or providing and dropping units (open monad'').

    92     \item The type of states may change due to a transformation.

    93   \end{itemize}

    94 *}

    95

    96

    97 subsection {* Monad laws *}

    98

    99 text {*

   100   The common monadic laws hold and may also be used

   101   as normalization rules for monadic expressions:

   102 *}

   103

   104 lemmas monad_simp = Pair_scomp scomp_Pair id_fcomp fcomp_id

   105   scomp_scomp scomp_fcomp fcomp_scomp fcomp_assoc

   106

   107 text {*

   108   Evaluation of monadic expressions by force:

   109 *}

   110

   111 lemmas monad_collapse = monad_simp fcomp_apply scomp_apply split_beta

   112

   113

   114 subsection {* Do-syntax *}

   115

   116 setup {*

   117   Adhoc_Overloading.add_variant @{const_name bind} @{const_name scomp}

   118 *}

   119

   120

   121 text {*

   122   For an example, see HOL/Extraction/Higman.thy.

   123 *}

   124

   125 end