On Jun 1, 2017, at 1= 0:23 AM, Thierry Coquand <Thierry...@cse.gu.se> wrote:

If we are only interested in providing one -particul= ar- model of HITs,      the paper
on  cubical type  theory describes a way to  = ;interpret HIT together with a univalent
universe which is stable by HIT operations. This gives in p= articular the consistency
and the proof theoretic power of this extension of type the= ory.

but the Kan simplicial set model already does this = =E2=80=94 right?
don=E2=80=99t get me wrong =E2=80=94 I love the = cubes, and they have lots of nice properties for models of HoTT
=
=E2=80=94 but there was never really a question of the consistency or = coherence of simple HITs like propositional truncation or suspension.
=

the advance in the L-S paper is to give a ge= neral scheme for defining HITs syntactically
(a definition,= if you like, of what a HIT is, rather than a family of examples),
and then a general description of the semantics of these,
in a range of models of the basic theory.

<= /div>
Steve

The approach uses an operation of  =E2=80=9Cfla= ttening an open box=E2=80=9D, which solves in
this case the issue of interpreting HIT with parameters (su= ch as   propositional
truncation or suspension) without any coherence issue.
Since the syntax used in this paper is so close to the sema= ntics,  we limited
ourselves  to a syntactical presentation of this inter= pretation. But it can directly
be transformed to a semantical interpretation, as explained= in the following note
(which also incorporates a nice simplification of the opera= tion of flattering
an open box noticed by my coauthors). I also try to make mo= re explicit in the note
what is the problem solved by the =E2=80=9Cflattening boxes= =E2=80=9D method.

Only the cases of the spheres and propositional trunc= ation are described, but one
would expect the method to generalise to other HITs covered= e.g. in the HoTT book.

On 25 May 2017, at 20:25, Michael Shulman <shu...@sandiego.edu> wrote:
The following long-awaited paper is now available:

Semantics of higher inductive types
Peter LeFanu Lumsdaine, Mike Shulman
https://arxiv.org/a= bs/1705.07088

>From the abstract:

We introduce the notion of *cell monad with parameters*: a
semantically-defined scheme for specifying homotopically well-behaved
notions of structure. We then show that any suitable model category
has *weakly stable typal initial algebras* for any cell monad with
parameters. When combined with the local universes construction to
obtain strict stability, this specializes to give models of specific
higher inductive types, including spheres, the torus, pushout types,
truncations, the James construction, and general localisations.

Our results apply in any sufficiently nice Quillen model category,
including any right proper simplicial Cisinski model category (such as
simplicial sets) and any locally presentable locally cartesian closed
category (such as sets) with its trivial model structure. In
particular, any locally presentable locally cartesian closed
(=E2=88=9E,1)-category is presented by some model category to which our
results apply.

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