Semantics encoding

Properties

An informal notion of translation is not sufficient to help determine expressivity of languages, as it permits trivial encodings such as mapping all elements of A to the same element of B. Therefore, it is necessary to determine the definition of a "good enough" encoding. This notion varies with the application.

Commonly, an encoding [ ⋅ ] : A ⟶ B is expected to preserve a number of properties.

Preservation of compositions

soundness 

For every n-ary operator o p A of A, there exists an n-ary operator o p B of B such that ∀ T A 1 , T A 2 , … , T A n , [ o p A ( T A 1 , T A 2 , ⋯ , T A n ) ] = o p B ( [ T A 1 ] , [ T A 2 ] , ⋯ , [ T A n ] )

completeness 

For every n-ary operator o p A of A, there exists an n-ary operator o p B of B such that ∀ T B 1 , T B 2 , … , T B n , ∃ T A 1 , … , T A n , o p B ( T B 1 , ⋯ , T B N ) = [ o p A ( T A 1 , T A 2 , ⋯ , T A n ) ]

(Note: as far as the author is aware of, this criterion of completeness is never used.)

Preservation of compositions is useful insofar as it guarantees that components can be examined either separately or together without "breaking" any interesting property. In particular, in the case of compilations, this soundness guarantees the possibility of proceeding with separate compilation of components, while completeness guarantees the possibility of de-compilation.

Preservation of reductions

This assumes the existence of a notion of reduction on both language A and language B. Typically, in the case of a programming language, reduction is the relation which models the execution of a program.

We write ⟶ for one step of reduction and ⟶ ∗ for any number of steps of reduction.

soundness 

For every terms T A 1 , T A 2 of language A, if T A 1 ⟶ ∗ T A 2 then [ T A 1 ] ⟶ ∗ [ T A 2 ] .

completeness 

For every term T A 1 of language A and every terms T B 2 of language B, if [ T A 1 ] ⟶ ∗ T B 2 then there exists some T A 2 such that T B 2 = [ T A 2 ] .

This preservation guarantees that both languages behave the same way. Soundness guarantees that all possible behaviours are preserved while completeness guarantees that no behaviour is added by the encoding. In particular, in the case of compilation of a programming language, soundness and completeness together mean that the compiled program behaves accordingly to the high-level semantics of the programming language.

Preservation of termination

This also assumes the existence of a notion of reduction on both language A and language B.

soundness 

for any term T A , if all reductions of T A converge, then all reductions of [ T A ] converge.

completeness 

for any term [ T A ] , if all reductions of [ T A ] converge, then all reductions of T A converge.

In the case of compilation of a programming language, soundness guarantees that the compilation does not introduce non-termination such as endless loops or endless recursions. The completeness property is useful when language B is used to study or test a program written in language A, possibly by extracting key parts of the code: if this study or test proves that the program terminates in B, then it also terminates in A.

Preservation of observations

This assumes the existence of a notion of observation on both language A and language B. In programming languages, typical observables are results of inputs and outputs, by opposition to pure computation. In a description language such as HTML, a typical observable is the result of page rendering.

soundness 

for every observable o b s A on terms of A, there exists an observable o b s B of terms of B such that for any term T A with observable o b s A , [ T A ] has observable o b s B .

completeness 

for every observable o b s A on terms of A, there exists an observable o b s B on terms of B such that for any term [ T A ] with observable o b s B , T A has observable o b s A .

Preservation of simulations

This assumes the existence of notion of simulation on both language A and language B. In a programming languages, a program simulates another if it can perform all the same (observable) tasks and possibly some others. Simulations are used typically to describe compile-time optimizations.

soundness 

for every terms T A 1 , T A 2 , if T A 2 simulates T A 1 then [ T A 2 ] simulates [ T A 1 ] .

completeness 

for every terms T A 1 , T A 2 , if [ T A 2 ] simulates [ T A 1 ] then T A 2 simulates T A 1 .

Preservation of simulations is a much stronger property than preservation of observations, which it entails. In turn, it is weaker than a property of preservation of bisimulations. As in previous cases, soundness is important for compilation, while completeness is useful for testing or proving properties.

Preservation of equivalences

This assumes the existence of a notion of equivalence on both language A and language B. Typically, this can be a notion of equality of structured data or a notion of syntactically different yet semantically identical programs, such as structural congruence or structural equivalence.

soundness 

if two terms T A 1 and T A 2 are equivalent in A, then [ T A 1 ] and [ T A 2 ] are equivalent in B.

completeness 

if two terms [ T A 1 ] and [ T A 2 ] are equivalent in B, then T A 1 and T A 2 are equivalent in A.

Preservation of distribution

This assumes the existence of a notion of distribution on both language A and language B. Typically, for compilation of distributed programs written in Acute, JoCaml or E, this means distribution of processes and data among several computers or CPUs.

soundness 

if a term T A is the composition of two agents T A 1   |   T A 2 then [ T A ] must be the composition of two agents [ T A 1 ]   |   [ T A 2 ] .

completeness 

if a term [ T A ] is the composition of two agents T B 1   |   T B 2 then T B must be the composition of two agents T A 1   |   T A 2 such that [ T A 1 ] = T B 1 and [ T A 2 ] = T B 2 .