Concepts de base
Affine string-to-string functions definable in the planar affine λ-calculus λ℘ and first-order string transductions coincide.
Résumé
The paper investigates the expressiveness of the planar affine λ-calculus λ℘ in computing string-to-string functions. It proves that the class of affine λ℘-definable string-to-string functions is exactly the class of first-order string transductions.
The key insights are:
- λ℘-transducers, which are λ℘-terms of a specific form, can be interpreted as two-way planar reversible finite transducers (2PRFTs), a variant of two-way transducers that capture first-order transductions.
- This interpretation is done by defining an interpretation of purely affine λ℘-terms in the category of planar diagrams TransDiagΓ, which is shown to be a strict monoidal-closed poset-enriched category.
- The interpretation maps β-reductions to inequalities in TransDiagΓ, allowing to show that every λ℘-transducer can be compiled into an equivalent 2PRFT.
- The converse direction, that every first-order transduction is λ℘-definable, follows from a Krohn-Rhodes-style decomposition lemma and the closure properties of affine λ℘-definable functions.