author  wenzelm 
Wed, 27 Mar 2013 16:38:25 +0100  
changeset 51553  63327f679cff 
parent 51143  0a2371e7ced3 
child 53015  a1119cf551e8 
permissions  rwrr 
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theory Classes 
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imports Main Setup 
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begin 
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section {* Introduction *} 

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text {* 

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Type classes were introduced by Wadler and Blott \cite{wadler89how} 
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into the Haskell language to allow for a reasonable implementation 

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of overloading\footnote{throughout this tutorial, we are referring 
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to classical Haskell 1.0 type classes, not considering later 
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additions in expressiveness}. As a canonical example, a polymorphic 

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equality function @{text "eq \<Colon> \<alpha> \<Rightarrow> \<alpha> \<Rightarrow> bool"} which is overloaded on 

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different types for @{text "\<alpha>"}, which is achieved by splitting 

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introduction of the @{text eq} function from its overloaded 

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definitions by means of @{text class} and @{text instance} 

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declarations: \footnote{syntax here is a kind of isabellized 

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Haskell} 

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\begin{quote} 
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\noindent@{text "class eq where"} \\ 
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\hspace*{2ex}@{text "eq \<Colon> \<alpha> \<Rightarrow> \<alpha> \<Rightarrow> bool"} 
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\medskip\noindent@{text "instance nat \<Colon> eq where"} \\ 
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\hspace*{2ex}@{text "eq 0 0 = True"} \\ 

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\hspace*{2ex}@{text "eq 0 _ = False"} \\ 

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\hspace*{2ex}@{text "eq _ 0 = False"} \\ 

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\hspace*{2ex}@{text "eq (Suc n) (Suc m) = eq n m"} 

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\medskip\noindent@{text "instance (\<alpha>\<Colon>eq, \<beta>\<Colon>eq) pair \<Colon> eq where"} \\ 
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\hspace*{2ex}@{text "eq (x1, y1) (x2, y2) = eq x1 x2 \<and> eq y1 y2"} 
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\medskip\noindent@{text "class ord extends eq where"} \\ 
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\hspace*{2ex}@{text "less_eq \<Colon> \<alpha> \<Rightarrow> \<alpha> \<Rightarrow> bool"} \\ 

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\hspace*{2ex}@{text "less \<Colon> \<alpha> \<Rightarrow> \<alpha> \<Rightarrow> bool"} 

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\end{quote} 
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\noindent Type variables are annotated with (finitely many) classes; 

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these annotations are assertions that a particular polymorphic type 
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provides definitions for overloaded functions. 

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Indeed, type classes not only allow for simple overloading but form 
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a generic calculus, an instance of ordersorted algebra 

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\cite{nipkowsorts93,NipkowPrehofer:1993,Wenzel:1997:TPHOL}. 

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From a software engineering point of view, type classes roughly 
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correspond to interfaces in objectoriented languages like Java; so, 

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it is naturally desirable that type classes do not only provide 

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functions (class parameters) but also state specifications 

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implementations must obey. For example, the @{text "class eq"} 
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above could be given the following specification, demanding that 
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@{text "class eq"} is an equivalence relation obeying reflexivity, 

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symmetry and transitivity: 

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\begin{quote} 
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\noindent@{text "class eq where"} \\ 
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\hspace*{2ex}@{text "eq \<Colon> \<alpha> \<Rightarrow> \<alpha> \<Rightarrow> bool"} \\ 

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@{text "satisfying"} \\ 

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\hspace*{2ex}@{text "refl: eq x x"} \\ 

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\hspace*{2ex}@{text "sym: eq x y \<longleftrightarrow> eq x y"} \\ 

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\hspace*{2ex}@{text "trans: eq x y \<and> eq y z \<longrightarrow> eq x z"} 

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\end{quote} 

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\noindent From a theoretical point of view, type classes are 
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lightweight modules; Haskell type classes may be emulated by SML 

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functors \cite{classes_modules}. Isabelle/Isar offers a discipline 

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of type classes which brings all those aspects together: 

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\begin{enumerate} 

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\item specifying abstract parameters together with 
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corresponding specifications, 
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\item instantiating those abstract parameters by a particular 
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type 
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\item in connection with a ``less adhoc'' approach to overloading, 

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\item with a direct link to the Isabelle module system: 
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locales \cite{kammuellerlocales}. 

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\end{enumerate} 
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\noindent Isar type classes also directly support code generation in 
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a Haskell like fashion. Internally, they are mapped to more 

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primitive Isabelle concepts \cite{HaftmannWenzel:2006:classes}. 

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This tutorial demonstrates common elements of structured 
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specifications and abstract reasoning with type classes by the 

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algebraic hierarchy of semigroups, monoids and groups. Our 

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background theory is that of Isabelle/HOL \cite{isatutorial}, for 

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which some familiarity is assumed. 

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*} 
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section {* A simple algebra example \label{sec:example} *} 
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subsection {* Class definition *} 

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text {* 

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Depending on an arbitrary type @{text "\<alpha>"}, class @{text 

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"semigroup"} introduces a binary operator @{text "(\<otimes>)"} that is 
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assumed to be associative: 
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*} 

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class %quote semigroup = 
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fixes mult :: "\<alpha> \<Rightarrow> \<alpha> \<Rightarrow> \<alpha>" (infixl "\<otimes>" 70) 
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assumes assoc: "(x \<otimes> y) \<otimes> z = x \<otimes> (y \<otimes> z)" 

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text {* 

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\noindent This @{command class} specification consists of two parts: 
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the \qn{operational} part names the class parameter (@{element 

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"fixes"}), the \qn{logical} part specifies properties on them 

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(@{element "assumes"}). The local @{element "fixes"} and @{element 

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"assumes"} are lifted to the theory toplevel, yielding the global 

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parameter @{term [source] "mult \<Colon> \<alpha>\<Colon>semigroup \<Rightarrow> \<alpha> \<Rightarrow> \<alpha>"} and the 
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global theorem @{fact "semigroup.assoc:"}~@{prop [source] "\<And>x y z \<Colon> 
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\<alpha>\<Colon>semigroup. (x \<otimes> y) \<otimes> z = x \<otimes> (y \<otimes> z)"}. 

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*} 
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subsection {* Class instantiation \label{sec:class_inst} *} 

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text {* 

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The concrete type @{typ int} is made a @{class semigroup} instance 
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by providing a suitable definition for the class parameter @{text 

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"(\<otimes>)"} and a proof for the specification of @{fact assoc}. This is 

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accomplished by the @{command instantiation} target: 

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*} 
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instantiation %quote int :: semigroup 
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begin 

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definition %quote 
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mult_int_def: "i \<otimes> j = i + (j\<Colon>int)" 

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instance %quote proof 
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fix i j k :: int have "(i + j) + k = i + (j + k)" by simp 

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then show "(i \<otimes> j) \<otimes> k = i \<otimes> (j \<otimes> k)" 

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unfolding mult_int_def . 
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qed 
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end %quote 
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text {* 
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\noindent @{command instantiation} defines class parameters at a 
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particular instance using common specification tools (here, 

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@{command definition}). The concluding @{command instance} opens a 

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proof that the given parameters actually conform to the class 

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specification. Note that the first proof step is the @{method 

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default} method, which for such instance proofs maps to the @{method 

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intro_classes} method. This reduces an instance judgement to the 

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relevant primitive proof goals; typically it is the first method 

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applied in an instantiation proof. 

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From now on, the typechecker will consider @{typ int} as a @{class 
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semigroup} automatically, i.e.\ any general results are immediately 

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available on concrete instances. 

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\medskip Another instance of @{class semigroup} yields the natural 
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numbers: 

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*} 
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instantiation %quote nat :: semigroup 
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begin 

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primrec %quote mult_nat where 
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"(0\<Colon>nat) \<otimes> n = n" 

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 "Suc m \<otimes> n = Suc (m \<otimes> n)" 

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instance %quote proof 
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fix m n q :: nat 

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show "m \<otimes> n \<otimes> q = m \<otimes> (n \<otimes> q)" 

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by (induct m) auto 

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qed 

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end %quote 
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text {* 
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\noindent Note the occurence of the name @{text mult_nat} in the 
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primrec declaration; by default, the local name of a class operation 

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@{text f} to be instantiated on type constructor @{text \<kappa>} is 

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mangled as @{text f_\<kappa>}. In case of uncertainty, these names may be 

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inspected using the @{command "print_context"} command or the 

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corresponding ProofGeneral button. 

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*} 
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subsection {* Lifting and parametric types *} 
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text {* 

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Overloaded definitions given at a class instantiation may include 
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recursion over the syntactic structure of types. As a canonical 

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example, we model product semigroups using our simple algebra: 

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*} 
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instantiation %quote prod :: (semigroup, semigroup) semigroup 
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begin 
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definition %quote 
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mult_prod_def: "p\<^isub>1 \<otimes> p\<^isub>2 = (fst p\<^isub>1 \<otimes> fst p\<^isub>2, snd p\<^isub>1 \<otimes> snd p\<^isub>2)" 
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instance %quote proof 
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fix p\<^isub>1 p\<^isub>2 p\<^isub>3 :: "\<alpha>\<Colon>semigroup \<times> \<beta>\<Colon>semigroup" 
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show "p\<^isub>1 \<otimes> p\<^isub>2 \<otimes> p\<^isub>3 = p\<^isub>1 \<otimes> (p\<^isub>2 \<otimes> p\<^isub>3)" 

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unfolding mult_prod_def by (simp add: assoc) 
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qed 
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end %quote 
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text {* 
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\noindent Associativity of product semigroups is established using 
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the definition of @{text "(\<otimes>)"} on products and the hypothetical 
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associativity of the type components; these hypotheses are 
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legitimate due to the @{class semigroup} constraints imposed on the 

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type components by the @{command instance} proposition. Indeed, 

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this pattern often occurs with parametric types and type classes. 

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*} 
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subsection {* Subclassing *} 

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text {* 

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We define a subclass @{text monoidl} (a semigroup with a lefthand 
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neutral) by extending @{class semigroup} with one additional 

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parameter @{text neutral} together with its characteristic property: 

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*} 
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class %quote monoidl = semigroup + 
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fixes neutral :: "\<alpha>" ("\<one>") 

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assumes neutl: "\<one> \<otimes> x = x" 

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text {* 

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\noindent Again, we prove some instances, by providing suitable 
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parameter definitions and proofs for the additional specifications. 

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Observe that instantiations for types with the same arity may be 

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simultaneous: 

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*} 
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instantiation %quote nat and int :: monoidl 
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begin 

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definition %quote 
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neutral_nat_def: "\<one> = (0\<Colon>nat)" 

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definition %quote 
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neutral_int_def: "\<one> = (0\<Colon>int)" 

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instance %quote proof 
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fix n :: nat 

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show "\<one> \<otimes> n = n" 

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unfolding neutral_nat_def by simp 

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next 

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fix k :: int 

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show "\<one> \<otimes> k = k" 

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unfolding neutral_int_def mult_int_def by simp 

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qed 

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end %quote 
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instantiation %quote prod :: (monoidl, monoidl) monoidl 
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begin 
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definition %quote 
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neutral_prod_def: "\<one> = (\<one>, \<one>)" 

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instance %quote proof 
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fix p :: "\<alpha>\<Colon>monoidl \<times> \<beta>\<Colon>monoidl" 

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show "\<one> \<otimes> p = p" 

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unfolding neutral_prod_def mult_prod_def by (simp add: neutl) 

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qed 

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end %quote 
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text {* 
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\noindent Fullyfledged monoids are modelled by another subclass, 
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which does not add new parameters but tightens the specification: 
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*} 
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class %quote monoid = monoidl + 
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assumes neutr: "x \<otimes> \<one> = x" 

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instantiation %quote nat and int :: monoid 
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begin 

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instance %quote proof 
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fix n :: nat 

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show "n \<otimes> \<one> = n" 

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unfolding neutral_nat_def by (induct n) simp_all 

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next 

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fix k :: int 

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show "k \<otimes> \<one> = k" 

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unfolding neutral_int_def mult_int_def by simp 

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qed 

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end %quote 
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instantiation %quote prod :: (monoid, monoid) monoid 
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begin 
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instance %quote proof 
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fix p :: "\<alpha>\<Colon>monoid \<times> \<beta>\<Colon>monoid" 

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show "p \<otimes> \<one> = p" 

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unfolding neutral_prod_def mult_prod_def by (simp add: neutr) 

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qed 

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end %quote 
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text {* 
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\noindent To finish our small algebra example, we add a @{text 
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group} class with a corresponding instance: 

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*} 
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class %quote group = monoidl + 
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fixes inverse :: "\<alpha> \<Rightarrow> \<alpha>" ("(_\<div>)" [1000] 999) 

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assumes invl: "x\<div> \<otimes> x = \<one>" 

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instantiation %quote int :: group 
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begin 

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definition %quote 

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inverse_int_def: "i\<div> =  (i\<Colon>int)" 

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instance %quote proof 
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fix i :: int 

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have "i + i = 0" by simp 

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then show "i\<div> \<otimes> i = \<one>" 

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unfolding mult_int_def neutral_int_def inverse_int_def . 

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qed 

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end %quote 
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section {* Type classes as locales *} 
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subsection {* A look behind the scenes *} 
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text {* 
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The example above gives an impression how Isar type classes work in 
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practice. As stated in the introduction, classes also provide a 

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link to Isar's locale system. Indeed, the logical core of a class 

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is nothing other than a locale: 
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*} 
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class %quote idem = 
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fixes f :: "\<alpha> \<Rightarrow> \<alpha>" 
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assumes idem: "f (f x) = f x" 

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text {* 

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\noindent essentially introduces the locale 
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*} (*<*)setup %invisible {* Sign.add_path "foo" *} 
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(*>*) 

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locale %quote idem = 
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fixes f :: "\<alpha> \<Rightarrow> \<alpha>" 
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assumes idem: "f (f x) = f x" 

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text {* \noindent together with corresponding constant(s): *} 
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consts %quote f :: "\<alpha> \<Rightarrow> \<alpha>" 
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text {* 
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\noindent The connection to the type system is done by means 

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of a primitive type class 
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*} (*<*)setup %invisible {* Sign.add_path "foo" *} 
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(*>*) 

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classes %quote idem < type 
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(*<*)axiomatization where idem: "f (f (x::\<alpha>\<Colon>idem)) = f x" 
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setup %invisible {* Sign.parent_path *}(*>*) 
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text {* \noindent together with a corresponding interpretation: *} 
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interpretation %quote idem_class: 
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idem "f \<Colon> (\<alpha>\<Colon>idem) \<Rightarrow> \<alpha>" 
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(*<*)proof qed (rule idem)(*>*) 
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text {* 
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\noindent This gives you the full power of the Isabelle module system; 
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conclusions in locale @{text idem} are implicitly propagated 
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to class @{text idem}. 
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*} (*<*)setup %invisible {* Sign.parent_path *} 
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(*>*) 

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subsection {* Abstract reasoning *} 
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text {* 

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Isabelle locales enable reasoning at a general level, while results 
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are implicitly transferred to all instances. For example, we can 
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now establish the @{text "left_cancel"} lemma for groups, which 

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states that the function @{text "(x \<otimes>)"} is injective: 
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*} 
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lemma %quote (in group) left_cancel: "x \<otimes> y = x \<otimes> z \<longleftrightarrow> y = z" 
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proof 

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assume "x \<otimes> y = x \<otimes> z" 

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then have "x\<div> \<otimes> (x \<otimes> y) = x\<div> \<otimes> (x \<otimes> z)" by simp 

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then have "(x\<div> \<otimes> x) \<otimes> y = (x\<div> \<otimes> x) \<otimes> z" using assoc by simp 

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then show "y = z" using neutl and invl by simp 

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next 

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assume "y = z" 

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then show "x \<otimes> y = x \<otimes> z" by simp 

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qed 

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text {* 

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\noindent Here the \qt{@{keyword "in"} @{class group}} target 
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specification indicates that the result is recorded within that 

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context for later use. This local theorem is also lifted to the 

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global one @{fact "group.left_cancel:"} @{prop [source] "\<And>x y z \<Colon> 

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\<alpha>\<Colon>group. x \<otimes> y = x \<otimes> z \<longleftrightarrow> y = z"}. Since type @{text "int"} has been 

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made an instance of @{text "group"} before, we may refer to that 

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fact as well: @{prop [source] "\<And>x y z \<Colon> int. x \<otimes> y = x \<otimes> z \<longleftrightarrow> y = 

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z"}. 

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*} 
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subsection {* Derived definitions *} 
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text {* 

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Isabelle locales are targets which support local definitions: 
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*} 
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primrec %quote (in monoid) pow_nat :: "nat \<Rightarrow> \<alpha> \<Rightarrow> \<alpha>" where 
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"pow_nat 0 x = \<one>" 

419 
 "pow_nat (Suc n) x = x \<otimes> pow_nat n x" 

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text {* 

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\noindent If the locale @{text group} is also a class, this local 
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definition is propagated onto a global definition of @{term [source] 
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"pow_nat \<Colon> nat \<Rightarrow> \<alpha>\<Colon>monoid \<Rightarrow> \<alpha>\<Colon>monoid"} with corresponding theorems 

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@{thm pow_nat.simps [no_vars]}. 

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\noindent As you can see from this example, for local definitions 
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you may use any specification tool which works together with 

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locales, such as Krauss's recursive function package 

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\cite{krauss2006}. 
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*} 
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subsection {* A functor analogy *} 
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text {* 

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We introduced Isar classes by analogy to type classes in functional 
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programming; if we reconsider this in the context of what has been 

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said about type classes and locales, we can drive this analogy 

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further by stating that type classes essentially correspond to 

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functors that have a canonical interpretation as type classes. 

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There is also the possibility of other interpretations. For 

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example, @{text list}s also form a monoid with @{text append} and 

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@{term "[]"} as operations, but it seems inappropriate to apply to 

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lists the same operations as for genuinely algebraic types. In such 

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a case, we can simply make a particular interpretation of monoids 

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for lists: 

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*} 
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interpretation %quote list_monoid: monoid append "[]" 
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proof qed auto 
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text {* 

455 
\noindent This enables us to apply facts on monoids 

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to lists, e.g. @{thm list_monoid.neutl [no_vars]}. 

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When using this interpretation pattern, it may also 

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be appropriate to map derived definitions accordingly: 

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*} 

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primrec %quote replicate :: "nat \<Rightarrow> \<alpha> list \<Rightarrow> \<alpha> list" where 
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"replicate 0 _ = []" 

464 
 "replicate (Suc n) xs = xs @ replicate n xs" 

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interpretation %quote list_monoid: monoid append "[]" where 
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"monoid.pow_nat append [] = replicate" 
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proof  

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interpret monoid append "[]" .. 
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show "monoid.pow_nat append [] = replicate" 
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proof 

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fix n 

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show "monoid.pow_nat append [] n = replicate n" 

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by (induct n) auto 

475 
qed 

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qed intro_locales 

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text {* 
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\noindent This pattern is also helpful to reuse abstract 

480 
specifications on the \emph{same} type. For example, think of a 

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class @{text preorder}; for type @{typ nat}, there are at least two 

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possible instances: the natural order or the order induced by the 

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divides relation. But only one of these instances can be used for 

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@{command instantiation}; using the locale behind the class @{text 

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preorder}, it is still possible to utilise the same abstract 

486 
specification again using @{command interpretation}. 

487 
*} 

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subsection {* Additional subclass relations *} 
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text {* 

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Any @{text "group"} is also a @{text "monoid"}; this can be made 
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explicit by claiming an additional subclass relation, together with 

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a proof of the logical difference: 

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*} 
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subclass %quote (in group) monoid 
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proof 
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fix x 
500 
from invl have "x\<div> \<otimes> x = \<one>" by simp 

501 
with assoc [symmetric] neutl invl have "x\<div> \<otimes> (x \<otimes> \<one>) = x\<div> \<otimes> x" by simp 

502 
with left_cancel show "x \<otimes> \<one> = x" by simp 

503 
qed 

23956  504 

505 
text {* 

38812  506 
The logical proof is carried out on the locale level. Afterwards it 
507 
is propagated to the type system, making @{text group} an instance 

508 
of @{text monoid} by adding an additional edge to the graph of 

509 
subclass relations (\figref{fig:subclass}). 

25247  510 

511 
\begin{figure}[htbp] 

512 
\begin{center} 

513 
\small 

514 
\unitlength 0.6mm 

515 
\begin{picture}(40,60)(0,0) 

516 
\put(20,60){\makebox(0,0){@{text semigroup}}} 

517 
\put(20,40){\makebox(0,0){@{text monoidl}}} 

518 
\put(00,20){\makebox(0,0){@{text monoid}}} 

519 
\put(40,00){\makebox(0,0){@{text group}}} 

520 
\put(20,55){\vector(0,1){10}} 

521 
\put(15,35){\vector(1,1){10}} 

522 
\put(25,35){\vector(1,3){10}} 

523 
\end{picture} 

524 
\hspace{8em} 

525 
\begin{picture}(40,60)(0,0) 

526 
\put(20,60){\makebox(0,0){@{text semigroup}}} 

527 
\put(20,40){\makebox(0,0){@{text monoidl}}} 

528 
\put(00,20){\makebox(0,0){@{text monoid}}} 

529 
\put(40,00){\makebox(0,0){@{text group}}} 

530 
\put(20,55){\vector(0,1){10}} 

531 
\put(15,35){\vector(1,1){10}} 

532 
\put(05,15){\vector(3,1){30}} 

533 
\end{picture} 

534 
\caption{Subclass relationship of monoids and groups: 

535 
before and after establishing the relationship 

30134  536 
@{text "group \<subseteq> monoid"}; transitive edges are left out.} 
25247  537 
\label{fig:subclass} 
538 
\end{center} 

539 
\end{figure} 

30227  540 

38812  541 
For illustration, a derived definition in @{text group} using @{text 
542 
pow_nat} 

23956  543 
*} 
544 

28565  545 
definition %quote (in group) pow_int :: "int \<Rightarrow> \<alpha> \<Rightarrow> \<alpha>" where 
546 
"pow_int k x = (if k >= 0 

547 
then pow_nat (nat k) x 

548 
else (pow_nat (nat ( k)) x)\<div>)" 

23956  549 

550 
text {* 

38812  551 
\noindent yields the global definition of @{term [source] "pow_int \<Colon> 
552 
int \<Rightarrow> \<alpha>\<Colon>group \<Rightarrow> \<alpha>\<Colon>group"} with the corresponding theorem @{thm 

553 
pow_int_def [no_vars]}. 

24991  554 
*} 
23956  555 

25868  556 
subsection {* A note on syntax *} 
557 

558 
text {* 

38812  559 
As a convenience, class context syntax allows references to local 
560 
class operations and their global counterparts uniformly; type 

561 
inference resolves ambiguities. For example: 

25868  562 
*} 
563 

28565  564 
context %quote semigroup 
25868  565 
begin 
566 

28565  567 
term %quote "x \<otimes> y"  {* example 1 *} 
568 
term %quote "(x\<Colon>nat) \<otimes> y"  {* example 2 *} 

25868  569 

28566  570 
end %quote 
25868  571 

28565  572 
term %quote "x \<otimes> y"  {* example 3 *} 
25868  573 

574 
text {* 

38812  575 
\noindent Here in example 1, the term refers to the local class 
576 
operation @{text "mult [\<alpha>]"}, whereas in example 2 the type 

577 
constraint enforces the global class operation @{text "mult [nat]"}. 

578 
In the global context in example 3, the reference is to the 

579 
polymorphic global class operation @{text "mult [?\<alpha> \<Colon> semigroup]"}. 

25868  580 
*} 
22347  581 

29705  582 
section {* Further issues *} 
583 

584 
subsection {* Type classes and code generation *} 

22317  585 

586 
text {* 

38812  587 
Turning back to the first motivation for type classes, namely 
588 
overloading, it is obvious that overloading stemming from @{command 

589 
class} statements and @{command instantiation} targets naturally 

590 
maps to Haskell type classes. The code generator framework 

591 
\cite{isabellecodegen} takes this into account. If the target 

592 
language (e.g.~SML) lacks type classes, then they are implemented by 

593 
an explicit dictionary construction. As example, let's go back to 

594 
the power function: 

22317  595 
*} 
596 

28565  597 
definition %quote example :: int where 
598 
"example = pow_int 10 (2)" 

22317  599 

600 
text {* 

31691  601 
\noindent This maps to Haskell as follows: 
22317  602 
*} 
39743  603 
text %quotetypewriter {* 
39680  604 
@{code_stmts example (Haskell)} 
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*} 
22317  606 

607 
text {* 

31691  608 
\noindent The code in SML has explicit dictionary passing: 
22317  609 
*} 
39743  610 
text %quotetypewriter {* 
39680  611 
@{code_stmts example (SML)} 
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*} 
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20946  614 

38812  615 
text {* 
616 
\noindent In Scala, implicts are used as dictionaries: 

617 
*} 

39743  618 
text %quotetypewriter {* 
39680  619 
@{code_stmts example (Scala)} 
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*} 
38812  621 

622 

29705  623 
subsection {* Inspecting the type class universe *} 
624 

625 
text {* 

38812  626 
To facilitate orientation in complex subclass structures, two 
627 
diagnostics commands are provided: 

29705  628 

629 
\begin{description} 

630 

631 
\item[@{command "print_classes"}] print a list of all classes 

632 
together with associated operations etc. 

633 

634 
\item[@{command "class_deps"}] visualizes the subclass relation 

635 
between all classes as a Hasse diagram. 

636 

637 
\end{description} 

638 
*} 

639 

20946  640 
end 
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641 