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theory Paper

imports "../Nominal/Test"

begin

notation (latex output)

  swap ("'(_ _')" [1000, 1000] 1000) and

  fresh ("_ # _" [51, 51] 50) and

  supp ("supp _" [78] 73) and

  uminus ("-_" [78] 73) and

  If  ("if _ then _ else _" 10)

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section {* Introduction *}

text {*

  It has not yet fared so well in the POPLmark challenge

  as the second part contain a formalisation of records 

  where ...

  The difficulty can be appreciated by considering that the

  definition given by Leroy in [] is incorrect (it omits a

  side-condition).

  Examples: type-schemes, Spi-calculus

  Contributions:  We provide definitions for when terms

  involving general bindings are alpha-equivelent.

*}

section {* A Short Review of the Nominal Logic Work *}

text {*

  At its core, Nominal Isabelle is based on the nominal logic work by Pitts

  \cite{Pitts03}. The central notions in this work are sorted atoms and

  permutations of atoms. The sorted atoms represent different

  kinds of variables, such as term- and type-variables in Core-Haskell, and it

  is assumed that there is an infinite supply of atoms for each sort. 

  However, in order to simplify the description of our work, we shall

  assume in this paper that there is only a single sort of atoms.

  Permutations are bijective functions from atoms to atoms that are 

  the identity everywhere except on a finite number of atoms. There is a 

  two-place permutation operation written

  @{text [display,indent=5] "_ \<bullet> _  ::  (\<alpha> \<times> \<alpha>) list \<Rightarrow> \<beta> \<Rightarrow> \<beta>"}

  \noindent 

  with a generic type in which @{text "\<alpha>"} stands for the type of atoms 

  and @{text "\<beta>"} for the type of the objects on which the permutation 

  acts. In Nominal Isabelle the identity permutation is written as @{term "0::perm"},

  the composition of two permutations @{term p} and @{term q} as \mbox{@{term "p + q"}} 

  and the inverse permutation @{term p} as @{text "- p"}. The permutation

  operation is defined for products, lists, sets, functions, booleans etc 

  (see \cite{HuffmanUrban10}).

  The most original aspect of the nominal logic work of Pitts et al is a general

  definition for ``the set of free variables of an object @{text "x"}''.  This

  definition is general in the sense that it applies not only to lambda-terms,

  but also to lists, products, sets and even functions. The definition depends

  only on the permutation operation and on the notion of equality defined for

  the type of @{text x}, namely:

  @{thm [display,indent=5] supp_def[no_vars, THEN eq_reflection]}

  \noindent

  There is also the derived notion for when an atom @{text a} is \emph{fresh}

  for an @{text x}, defined as

  @{thm [display,indent=5] fresh_def[no_vars]}

  \noindent

  A striking consequence of these definitions is that we can prove

  without knowing anything about the structure of @{term x} that

  swapping two fresh atoms, say @{text a} and @{text b}, leave 

  @{text x} unchanged. For the proof we use the following lemma 

  about swappings applied to an @{text x}:

*}

section {* Abstractions *}

section {* Alpha-Equivalence and Free Variables *}

section {* Examples *}

section {* Conclusion *}

text {*

  \noindent

  {\bf Acknowledgements:} We thank Andrew Pitts for the many discussions

  about the topic. We thank Peter Sewell for making [] available 

  to us and explaining some of the finer points of the OTT tool.

*}

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end

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