[Hol-info] Software Implementation of the Mathematical Logic R0 Available for Download

[Ken K. <ma...@ke...>]

Dear Members of the Research Community,

I am pleased to announce that the software implementation of the mathematical 
logic R0, a further development of Peter B. Andrews' logic Q0, is now 
available. The syntactic features provided by R0 are type variables 
(polymorphic type theory), the binding of type variables with the abstraction 
operator and single variable binder λ (type abstraction), and (some of) the 
means necessary for dependent types (dependent type theory).

The software implementation can be downloaded (license restrictions apply) at
        http://doi.org/10.4444/100.10.3


The logic R0 does not only allow quantification over types with quantifiers, as 
specified in [Andrews, 1965] and [Melham, 1993b], but, moreover, the binding of 
type variables with lambda (type abstraction), as suggested by Mike Gordon for 
HOL: "[...] 'second order' [lambda]-terms like \a. \x:a. x, perhaps such terms 
should be included in the HOL logic." [Gordon, 2001, p. 22]

The expressiveness of the formal language obtained with type abstraction allows 
for a natural formulation of group theory [cf. p. 12 of 
http://doi.org/10.4444/100.10.2]. With the set (type) of Boolean values o, the 
exclusive disjunction XOR, and an appropriate definition of groups Grp [p. 
362], the fact that (o, XOR) is a group can be expressed in lambda notation 
with [p. 420]
        Grp o XOR

This enhancement of the expressiveness of the formal language overcomes the 
"limitation of the simple HOL type system [...] that there is no explicit 
quantifier over polymorphic type variables, which can make many standard 
results [...] awkward to express [...]. [...] For example, in one of the most 
impressive formalization efforts to date [Gonthier et al., 2013] the entire 
group theory framework is developed in terms of subsets of a single universe 
group, apparently to avoid the complications from groups with general and 
possibly heterogeneous types." [Harrison, Urban, and Wiedijk, 2014, pp. 170 f.]

While in both Andrews' Q0 and Gordon's HOL the universal quantifier is defined 
as
    ALL  :=  [\p. p = [\x.T] ]
[Andrews, 2002, p. 212; Gordon and Melham, 1993], in R0, with type abstraction, 
the type is made explicit:
    ALL  :=  [\t. [\p. p = [\x.T] ] ]
with p of type (ot), or t -> o [p. 359 of http://doi.org/10.4444/100.10.2].
Then, the set-theoretic proposition
    ALL n : NAT . n+1 > 0
in type theory can be expressed very naturally by
    ALL NAT [\n . n+1 > 0]

Furthermore, the enhanced expressiveness provided by R0 avoids the 
circumlocutions connected with preliminary solutions like axiomatic type 
classes recently developed and discussed for Isabelle/HOL. The expressiveness 
of type abstraction also replaces the notion of compound types, which in HOL 
are used for ordered pairs (the Cartesian product, cf. [Gordon and Melham, 
1993]), that in R0 can be formalized without compound types [cf. pp. 378 f. of 
http://doi.org/10.4444/100.10.2].


R0 has an intuitive method of type introduction, which does not require the 
additional axioms of the HOL type introduction mechanism: "Whenever a theorem 
of the form p{oa}e{a} is inferred [...] (which in set theory is expressed by e 
∈ p) [...] p is acknowledged as a type" [p. 11 of 
http://doi.org/10.4444/100.10.2].


Mike Gordon's HOL developed at Cambridge University is, like Andrews' logic Q0, 
based on the Simple Theory of Types (1940) developed by Alonzo Church, Andrews' 
Ph.D. advisor at Princeton University. Among the HOL group, there has always 
been the awareness that besides automation, there is the philosophical 
(logical) desire to reduce the means of the logic to a few principles. In the 
HOL handbook, Andrew M. Pitts wrote the legendary sentence: "From a logical 
point of view, it would be better to have a simpler substitution primitive, 
such as 'Rule R' of Andrews' logic Q0, and then to derive more complex rules 
from it." [Gordon and Melham, 1993, p. 213]

In the same spirit, Mike Gordon wrote on the genesis of HOL: "[T]he terms [...] 
could be encoded [...] in such a way that the LSM expansion-law just becomes a 
derived rule [...]. This approach is both more elegant and rests on a firmer 
logical foundation, so I switched to it and HOL was born." [Gordon, 2000, p. 
173]

The general principle of reducing the logic (including the language) to a few 
principles is the main criterion for the design of Q0 (having only a single 
primitive rule of inference, Rule R), which is summarized by Peter B. Andrews 
as follows: "Therefore we shall turn our attention to finding a formulation of 
type theory which is as expressive as possible, allowing mathematical ideas to 
be expressed precisely with a minimum of circumlocutions, and which is as 
simple and economical as is possible without sacrificing expressiveness. The 
reader will observe that the formal language we find arises very naturally from 
a few fundamental design decisions." [Andrews, 2002, pp. 205 f.]

R0 "follows Andrews' concept of expressiveness (I also use the term 
reducibility), which aims at the ideal and natural language of formal logic and 
mathematics." [p. 11 of http://doi.org/10.4444/100.10.2] Therefore R0 is, 
unlike most other implementations, a Hilbert-style system, opting for 
expressiveness instead of automation.

R0 implements the philosophical program of Russell's and Whitehead's 'Principia 
Mathematica', logicism, i.e., the reduction of mathematics to formal logic, and 
even more, generalizes this idea by reducing formal logic itself to a few 
principles.


Like John Harrison's HOL Light, R0 has an extremely small logical kernel. Being 
a Hilbert-style system, it has the smallest number of rules of inference among 
the programs implementing a fixed logic (not regarding logical frameworks with 
another kind of expressiveness). R0 resembles Norman Megill's Metamath, which 
"attempts to use the minimum possible framework needed to express mathematics 
and its proofs." [http://us.metamath.org/] With a size of less than 100 KB, it 
is the smallest proof checker or interactive theorem prover, including the 
current versions of John Harrison's HOL Light, Mark Adams' HOL Zero, Norman 
Megill's Metamath, and Freek Wiedijk's reimplementation of Automath.

Like Q0, R0 uses the description operator, avoiding the problems of the epsilon 
operator for HOL already discussed by Mike Gordon himself: "It must be admitted 
that the ε-operator looks rather suspicious." [Gordon, 2001, p. 24] "The 
inclusion of ε-terms into HOL 'builds in' the Axiom of Choice [...]." [Gordon, 
2001, p. 24]

R0 and PVS are the only implementations based on classical type theory with 
some form of dependent types. Also, R0 and PVS are the only implementations 
based on classical type theory with mathematical entities that may have 
different types (or which have at least some form of subtyping).

Unlike in Coq, in R0, no use is made of the Curry-Howard isomorphism, favoring 
a direct (unencoded, and hence, natural) expression rather than the encoding of 
proofs. For the same reason, it is an object (fixed) logic and not a logical 
framework (such as Larry Paulson's Isabelle and Norman Megill's Metamath). Like 
in Cris Perdue's Prooftoys [http://prooftoys.org, http://mathtoys.org] - a 
natural deduction variant of Andrews' Q0 - in R0, the turnstile symbol is 
replaced by the logical implication [p. 12 of http://doi.org/10.4444/100.10.2].

R0 is, together with HOL Zero [Adams, 2016, p. 34], the only proof checker or 
interactive theorem prover which has the property of Pollack-consistency, 
namely "being able to correctly parse formulas that it printed itself" 
[Wiedijk, 2012, p. 85]. R0 is the only proof checker or interactive theorem 
prover which can correctly parse whole proofs (and not only formulas) that it 
printed itself. Finally, R0 has the property of "faithfulness, where internal 
representation and concrete syntax correctly correspond. A printer that printed 
false as true and true as false might be Pollack-consistent but would not be 
faithful." [Adams, 2016, p. 21]

R0 is, like Automath, a mere proof checker (practically without any automation 
at all).


A full treatment of R0 shall be announced at
        http://doi.org/10.4444/100.10.1

For references, please see: http://doi.org/10.4444/100.111

The SHA-512 checksum for file 'R0-v1.0.0.tar.gz' is:
538e742c5d42258cf460e46d84c60ceb fa6b5843efba4f73c2c17206f5bab931
7f6edb31686b933147abd17af8114277 b833baf189f42c2b7587c409aecaed1f


Kind regards,

Ken Kubota

____________________

Ken Kubota
http://doi.org/10.4444/100