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    • CommentRowNumber1.
    • CommentAuthorMike Shulman
    • CommentTimeFeb 8th 2010

    Created W-type.

    • CommentRowNumber2.
    • CommentAuthorMike Shulman
    • CommentTimeFeb 8th 2010

    And a related query at pretopos.

    • CommentRowNumber3.
    • CommentAuthorUrs
    • CommentTimeSep 22nd 2012

    Added to W-type a section Properties with a pointer to Danielsson’s recent post.

    • CommentRowNumber4.
    • CommentAuthorUrs
    • CommentTimeJan 9th 2014

    added to W-type pointers to

    • the article by Gambino-Hyland on dependent W-types

    • the article by van den Berg and Moerdijk on W-types in model categories (those given by fibrations)

    Also made the following trivial edit: changed the name of the ambient category from CC to 𝒞\mathcal{C} as it seemed rather bad style to have a category named CC with objects named AA, BB and DD.

    I’ll have a question or comment on W-types in linear type theory, but I’ll put that in a separate thread…

    • CommentRowNumber5.
    • CommentAuthorTobyBartels
    • CommentTimeJan 18th 2014
    • (edited Jan 18th 2014)

    Heh, you changed the name from a recognizable letter to a nondescript grey box, according to my Firefox for Android. But that’s OK, I don't think that we should actually cater to that.

  1. Reference to Michael Abbott, Thorsten Altenkirch, and Neil Ghani, Representing Nested Inductive Types using W-types

    Anonymous

    diff, v29, current

    • CommentRowNumber7.
    • CommentAuthorUrs
    • CommentTimeJul 25th 2019
    • (edited Jul 25th 2019)

    Thanks for editing.

    Allow me to mention some hints on formatting references:

    • leave a whitespace to any previous text, not to have the output clutter

    • a * followed by a whitespace right at the beginning of the line makes a bullet item for the reference to fit into the rest of the list

    • enclosing author names in double square brackets hyperlinks them to their pages

    • explicit hyperlinks need the http://-prefix to be parsed

    I have slightly reformatted accordingly, now the source looks like so

      * [[Michael Abbott]], [[Thorsten Altenkirch]], and [[Neil Ghani]], 
      _Representing Nested Inductive Types using W-types_ 
      ([pdf](http://www.cs.nott.ac.uk/~psztxa/publ/icalp04.pdf))
    

    and renders like so:

    • CommentRowNumber8.
    • CommentAuthorspitters
    • CommentTimeJul 25th 2019
    Re #6. Thanks! I'll try to remember that.
    • CommentRowNumber9.
    • CommentAuthorMike Shulman
    • CommentTimeFeb 1st 2021

    Added reference

    diff, v32, current

    • CommentRowNumber10.
    • CommentAuthorMike Shulman
    • CommentTimeAug 13th 2021

    Spencer Breiner pointed out on Zulip that this page claims that natural numbers (and lists) are W-types, when type-theoretically they are actually not. In type theory, a W-type has only one constructor, while nat and lists have two constructors. They can be encoded as W-types using a sum-type to wrap up the two constructors into one, but the result doesn’t have the same computation rules and requires function extensionality for its correctness. So I removed these examples from this page and put the corresponding text at inductive type.

    diff, v33, current

    • CommentRowNumber11.
    • CommentAuthorDavid_Corfield
    • CommentTimeAug 13th 2021
    • (edited Aug 13th 2021)

    A W-type is a set or type which is defined inductively in a well-founded way based on a type of “constructors” and a type of “arities”. It is thus a particular kind of inductive type, with one constructor having a certain canonical form.

    How’s that “thus” working? Why “one constructor” in this concept.

    • CommentRowNumber12.
    • CommentAuthorMike Shulman
    • CommentTimeAug 14th 2021

    Maybe we should just omit “thus”. Would it make sense that way?

    • CommentRowNumber13.
    • CommentAuthorDavid_Corfield
    • CommentTimeAug 14th 2021

    Have written it as

    A W-type is a set or type which is defined inductively in a well-founded way based on a type of “constructors” and a type of “arities”, with one constructor having a certain canonical form. As such it is a particular kind of inductive type.

    The page still contains a query from you Mike:

    Is the term “W-type” still used in this generality? Or are they just called “inductive types”?

    Is this resolved yet?

    • CommentRowNumber14.
    • CommentAuthorMike Shulman
    • CommentTimeAug 15th 2021

    Clarified the situation with indices, parameters, and non-uniform parameters.

    diff, v35, current

    • CommentRowNumber15.
    • CommentAuthorMike Shulman
    • CommentTimeAug 15th 2021

    Thanks for pointing that out, have now resolved it.

    • CommentRowNumber16.
    • CommentAuthorUrs
    • CommentTimeDec 31st 2022
    • (edited Dec 31st 2022)

    I have added publication data to:

    I am guessing that this is the reference which was meant to be referred to at the end of this paragraph (where it had a broken anchor to [AAG](#AAG)) and so I fixed the link accordingly

    diff, v36, current

    • CommentRowNumber17.
    • CommentAuthorUrs
    • CommentTimeJan 1st 2023
    • (edited Jan 1st 2023)

    did a fair bit of rewording and adjustment in the Idea-section (here) for better (logical) flow

    switched to calligraphic “𝒲\mathcal{W}-type”, throughout

    switched the ordering of the subsections “In category theory”, “In type theory”.

    The 𝒲\mathcal{W}-type inference rules here had a query box comment:

    Andreas Abel: A and B are used exactly the other way round in the introductory text to this page. (Before, A was the arity, now B is the arity.) Harmonize!?

    Indeed. Am harmonizing this now by using, throughout:

    • CC” for the type of constructors

    • AA” for the type of arities.

    introduced formatting of 𝒲\mathcal{W}-type denotation as “𝒲c:CA(c)\underset{c \colon C}{\mathcal{W}} \, A(c)

    diff, v37, current

    • CommentRowNumber18.
    • CommentAuthorUrs
    • CommentTimeJan 1st 2023

    fixed these bibitems:

    diff, v37, current

    • CommentRowNumber19.
    • CommentAuthorUrs
    • CommentTimeJan 1st 2023

    adjusted the typesetting of the type inference rules, in particular:

    • added more widespace to make it easier on the eye to recognize the syntax tree branching

    • chose more suggestive term names

    now it looks as follows:


    (1) type formation rule:

    C:Type;c:CA(c):Type| || |𝒲c:CA(c):Type \frac { C \,\colon\, Type \;; \;\;\; c \,\colon\, C \;\;\vdash\;\; A(c) \,\colon\,Type \mathclap{\phantom{\vert_{\vert}}} } { \mathclap{\phantom{\vert^{\vert}}} \underset{c \colon C}{\mathcal{W}}\, A(c) \,\colon\, Type }

    (2) term introduction rule:

    root:C;subtr:A(c)𝒲c:CA(c)| |tree(root,subtr):𝒲c:CA(c) \frac{ \vdash\; root \,\colon\, C \;; \;\; subtr \,\colon\, A(c) \to \underset{c \colon C}{\mathcal{W}}\, A(c) }{ \mathclap{\phantom{\vert^{\vert}}} tree\big(root ,\, subtr\big) \,\colon\, \underset{c \colon C}{\mathcal{W}}\, A(c) }

    (3) term elimination rule:

    t:𝒲c:CA(c)D(t):Type; root:C,subt:A(c)𝒲c:CA(c),subt D:a:A(root)D(subt(a)) tree D(root,subt,subt D):D(tree(c,subt))| || |t:𝒲c:CA(c)wrec (D,tree D)(t):D(t) \frac{ \begin{array}{l} t \,\colon\, \underset{c \colon C}{\mathcal{W}}\, A(c) \;\vdash\; D(t) \,\colon\, Type \;; \\ root \,\colon\, C \,, \; subt \,\colon\, A(c) \to \underset{c \colon C}{\mathcal{W}}\, A(c) \,, \; subt_D \,\colon\, \underset{a \colon A(root)}{\prod} D\big(subt(a)\big) \\ \;\;\;\;\;\;\;\;\;\;\;\;\;\;\; \;\;\vdash\;\; tree_D\big(root,\,subt ,\, subt_D\big) \,\colon\, D\big(tree(c,\, subt)\big) \mathclap{\phantom{\vert_{\vert}}} \end{array} }{ \mathclap{\phantom{\vert^{\vert}}} t \,\colon\, \underset{c \colon C}{\mathcal{W}}\, A(c) \;\vdash\; wrec_{(D,tree_D)}(t) \,\colon\, D(t) }

    (4) computation rule:

    t:𝒲c:CA(c)D(t):Type; root:C,subt:A(c)𝒲c:CA(c),subt D:Πa:A(c)D(subt(a)) tree D(root,subt,subt D):D(tree(root,subt))| || |root:C,subtr:A(c)𝒲c:CA(c) wrec (D,tree D)(tree(root,subtr))=tree D(root,subt,λa.wrec (D,tree D)(subtr(a))) \frac{ \begin{array}{l} t \,\colon\, \underset{c \colon C}{\mathcal{W}}\, A(c) \;\vdash\; D(t) \,\colon\, Type \;; \\ root \,\colon\, C \,,\; subt \,\colon\, A(c) \to \underset{c \colon C}{\mathcal{W}}\, A(c) \,, \; subt_D \,\colon\, \underset{a\colon A(c)}{\Pi} D\big(subt(a)\big) \\ \;\;\;\;\;\;\;\;\;\;\;\;\;\;\; \;\;\vdash\;\; tree_D\big(root,\,subt,\,subt_D\big) \,\colon\, D\big(tree(root,\, subt)\big) \mathclap{\phantom{\vert_{\vert}}} \end{array} }{ \begin{array}{l} \mathclap{\phantom{\vert^{\vert}}} root \,\colon\, C \,, \;\; subtr \,\colon\, A(c) \to \underset{c \colon C}{\mathcal{W}}\, A(c) \\ \;\;\;\;\;\;\;\;\;\;\; \;\;\vdash\;\; wrec_{(D,tree_D)}\big( tree(root, \, subtr) \big) \;=\; tree_D \Big( root ,\, subt ,\, \lambda a . wrec_{(D,tree_D)}\big(subtr(a)\big) \Big) \end{array} }

    diff, v37, current

    • CommentRowNumber20.
    • CommentAuthorUrs
    • CommentTimeJan 1st 2023
    • (edited Jan 1st 2023)

    added pointer to:

    diff, v38, current

    • CommentRowNumber21.
    • CommentAuthorUrs
    • CommentTimeJan 1st 2023

    made explicit the example of the natural numbers W-type with a numbered environment (here)

    diff, v38, current

    • CommentRowNumber22.
    • CommentAuthorUrs
    • CommentTimeJan 3rd 2023

    organized all references on semantics of W-types by initial endofunctor algebras: here

    (cf. nForum announcement here)

    diff, v40, current

    • CommentRowNumber23.
    • CommentAuthorncfavier
    • CommentTimeMar 3rd 2023
    • (edited Mar 3rd 2023)

    mentioned that the dependent product is not dependent in the plain case by factoring through the reader monad

    diff, v42, current

    • CommentRowNumber24.
    • CommentAuthorDavidRoberts
    • CommentTimeMar 3rd 2023

    In the sentence

    More generally, endofunctors that look like polynomials in the traditional sense:

    F(Y)=A n×Y ×n++A 1×Y+A 0 F(Y) = A_n \times Y^{\times n} + \dots + A_1 \times Y + A_0

    can be constructed as polynomial endofunctors in the above sense in any Π\Pi-pretopos.

    I changed the Π\Pi-pretopos to just a pretopos, since that is all one needs for the finite products and coproducts.

    diff, v43, current

    • CommentRowNumber25.
    • CommentAuthorDavidRoberts
    • CommentTimeMar 6th 2023

    I plan to add in the morphism that makes the endofunctor in the previous comment polynomial, having now double checked it really works for an arbitrary pretopos. For a σ\sigma-pretopos (ie countable coproducts and corresponding extensivity) one gets analytic endofunctors (those that are formal power series) being polynomial for free as well. Then I’ll check about existence of W-types for these based just on having a parameterised NNO.

    • CommentRowNumber26.
    • CommentAuthorUrs
    • CommentTimeMay 18th 2023

    added earlier reference for the introduction of the notion of W-types:

    following a hint in another thread (here)

    diff, v47, current

  2. added earlier reference for the introduction of the notion of W-types:

    following a hint in another thread (here)

    Hubert Wasilewski

    diff, v48, current

    • CommentRowNumber28.
    • CommentAuthorUrs
    • CommentTime5 days ago

    In reality “Hubert Wasilewski” in #27 made no discernible edit.

  3. By why does the text of #27 copy your text from #26?

    • CommentRowNumber30.
    • CommentAuthorUrs
    • CommentTime4 days ago

    It keeps happening (I have been flagging it previously): Newbie users appear who announce an otherwise empty edit by copying my latest edit log.

    It is either a strange bug or a strange prank.

    I had meant to bring it to the attention of the the technical team, but didn’t get around to yet.

  4. If you try to edit without entering a name in the “Submit as ….” text field, the following things happen:

    1. A yellow box appears at the top of the page with the text “Please enter your name. (Due to a flood of low quality edits, we restrict anonymous edits.)”

    2. The comment of the last edit made on that page is copied into the comment box at the bottom of the page above the “Submit as ….” text field. Sometimes this is still empty because the last editor did not make a comment.

    • CommentRowNumber32.
    • CommentAuthorUrs
    • CommentTime4 days ago

    Thanks, that’s useful to know! I have brought this to the attention of the technical team.

  5. The comment of the last edit made on that page is copied into the comment box at the bottom of the page above the “Submit as ….” text field. Sometimes this is still empty because the last editor did not make a comment.

    Apparently, that was how the announcement box was designed. It worked as intended when the failed edit itself contained an announcement message (then that one would be displayed). I think this bug just showed more now because there are more ways for an edit to fail (e.g., empty username).

    I made a change that should fix this. Thanks for letting us now.

  6. Now, when you try to edit without entering a name in the “Submit as ….” text field and encounter the yellow text box at the top, the previous editor’s comments no longer appear in the comments box. However, the contents of the article disappear from the article text’s box as well.