(* Title: HOL/Library/State_Monad.thy
Author: Florian Haftmann, TU Muenchen
*)
header {* Combinator syntax for generic, open state monads (single threaded monads) *}
theory State_Monad
imports Main
begin
subsection {* Motivation *}
text {*
The logic HOL has no notion of constructor classes, so
it is not possible to model monads the Haskell way
in full genericity in Isabelle/HOL.
However, this theory provides substantial support for
a very common class of monads: \emph{state monads}
(or \emph{single-threaded monads}, since a state
is transformed single-threaded).
To enter from the Haskell world,
\url{http://www.engr.mun.ca/~theo/Misc/haskell_and_monads.htm}
makes a good motivating start. Here we just sketch briefly
how those monads enter the game of Isabelle/HOL.
*}
subsection {* State transformations and combinators *}
text {*
We classify functions operating on states into two categories:
\begin{description}
\item[transformations]
with type signature @{text "\<sigma> \<Rightarrow> \<sigma>'"},
transforming a state.
\item[``yielding'' transformations]
with type signature @{text "\<sigma> \<Rightarrow> \<alpha> \<times> \<sigma>'"},
``yielding'' a side result while transforming a state.
\item[queries]
with type signature @{text "\<sigma> \<Rightarrow> \<alpha>"},
computing a result dependent on a state.
\end{description}
By convention we write @{text "\<sigma>"} for types representing states
and @{text "\<alpha>"}, @{text "\<beta>"}, @{text "\<gamma>"}, @{text "\<dots>"}
for types representing side results. Type changes due
to transformations are not excluded in our scenario.
We aim to assert that values of any state type @{text "\<sigma>"}
are used in a single-threaded way: after application
of a transformation on a value of type @{text "\<sigma>"}, the
former value should not be used again. To achieve this,
we use a set of monad combinators:
*}
notation fcomp (infixl "o>" 60)
notation (xsymbols) fcomp (infixl "o>" 60)
notation scomp (infixl "o->" 60)
notation (xsymbols) scomp (infixl "o\<rightarrow>" 60)
abbreviation (input)
"return \<equiv> Pair"
text {*
Given two transformations @{term f} and @{term g}, they
may be directly composed using the @{term "op o>"} combinator,
forming a forward composition: @{prop "(f o> g) s = f (g s)"}.
After any yielding transformation, we bind the side result
immediately using a lambda abstraction. This
is the purpose of the @{term "op o\<rightarrow>"} combinator:
@{prop "(f o\<rightarrow> (\<lambda>x. g)) s = (let (x, s') = f s in g s')"}.
For queries, the existing @{term "Let"} is appropriate.
Naturally, a computation may yield a side result by pairing
it to the state from the left; we introduce the
suggestive abbreviation @{term return} for this purpose.
The most crucial distinction to Haskell is that we do
not need to introduce distinguished type constructors
for different kinds of state. This has two consequences:
\begin{itemize}
\item The monad model does not state anything about
the kind of state; the model for the state is
completely orthogonal and may be
specified completely independently.
\item There is no distinguished type constructor
encapsulating away the state transformation, i.e.~transformations
may be applied directly without using any lifting
or providing and dropping units (``open monad'').
\item The type of states may change due to a transformation.
\end{itemize}
*}
subsection {* Monad laws *}
text {*
The common monadic laws hold and may also be used
as normalization rules for monadic expressions:
*}
lemmas monad_simp = Pair_scomp scomp_Pair id_fcomp fcomp_id
scomp_scomp scomp_fcomp fcomp_scomp fcomp_assoc
text {*
Evaluation of monadic expressions by force:
*}
lemmas monad_collapse = monad_simp fcomp_apply scomp_apply split_beta
subsection {* Syntax *}
text {*
We provide a convenient do-notation for monadic expressions
well-known from Haskell. @{const Let} is printed
specially in do-expressions.
*}
nonterminals do_expr
syntax
"_do" :: "do_expr \<Rightarrow> 'a"
("do _ done" [12] 12)
"_scomp" :: "pttrn \<Rightarrow> 'a \<Rightarrow> do_expr \<Rightarrow> do_expr"
("_ <- _;// _" [1000, 13, 12] 12)
"_fcomp" :: "'a \<Rightarrow> do_expr \<Rightarrow> do_expr"
("_;// _" [13, 12] 12)
"_let" :: "pttrn \<Rightarrow> 'a \<Rightarrow> do_expr \<Rightarrow> do_expr"
("let _ = _;// _" [1000, 13, 12] 12)
"_done" :: "'a \<Rightarrow> do_expr"
("_" [12] 12)
syntax (xsymbols)
"_scomp" :: "pttrn \<Rightarrow> 'a \<Rightarrow> do_expr \<Rightarrow> do_expr"
("_ \<leftarrow> _;// _" [1000, 13, 12] 12)
translations
"_do f" => "f"
"_scomp x f g" => "f o\<rightarrow> (\<lambda>x. g)"
"_fcomp f g" => "f o> g"
"_let x t f" => "CONST Let t (\<lambda>x. f)"
"_done f" => "f"
print_translation {*
let
fun dest_abs_eta (Abs (abs as (_, ty, _))) =
let
val (v, t) = Syntax.variant_abs abs;
in (Free (v, ty), t) end
| dest_abs_eta t =
let
val (v, t) = Syntax.variant_abs ("", dummyT, t $ Bound 0);
in (Free (v, dummyT), t) end;
fun unfold_monad (Const (@{const_syntax scomp}, _) $ f $ g) =
let
val (v, g') = dest_abs_eta g;
in Const ("_scomp", dummyT) $ v $ f $ unfold_monad g' end
| unfold_monad (Const (@{const_syntax fcomp}, _) $ f $ g) =
Const ("_fcomp", dummyT) $ f $ unfold_monad g
| unfold_monad (Const (@{const_syntax Let}, _) $ f $ g) =
let
val (v, g') = dest_abs_eta g;
in Const ("_let", dummyT) $ v $ f $ unfold_monad g' end
| unfold_monad (Const (@{const_syntax Pair}, _) $ f) =
Const ("return", dummyT) $ f
| unfold_monad f = f;
fun contains_scomp (Const (@{const_syntax scomp}, _) $ _ $ _) = true
| contains_scomp (Const (@{const_syntax fcomp}, _) $ _ $ t) =
contains_scomp t
| contains_scomp (Const (@{const_syntax Let}, _) $ _ $ Abs (_, _, t)) =
contains_scomp t;
fun scomp_monad_tr' (f::g::ts) = list_comb
(Const ("_do", dummyT) $ unfold_monad (Const (@{const_syntax scomp}, dummyT) $ f $ g), ts);
fun fcomp_monad_tr' (f::g::ts) = if contains_scomp g then list_comb
(Const ("_do", dummyT) $ unfold_monad (Const (@{const_syntax fcomp}, dummyT) $ f $ g), ts)
else raise Match;
fun Let_monad_tr' (f :: (g as Abs (_, _, g')) :: ts) = if contains_scomp g' then list_comb
(Const ("_do", dummyT) $ unfold_monad (Const (@{const_syntax Let}, dummyT) $ f $ g), ts)
else raise Match;
in [
(@{const_syntax scomp}, scomp_monad_tr'),
(@{const_syntax fcomp}, fcomp_monad_tr'),
(@{const_syntax Let}, Let_monad_tr')
] end;
*}
text {*
For an example, see HOL/Extraction/Higman.thy.
*}
end