From f3ea5a492954c79fb3c555b836e512925884f2db Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Georges=20Dup=C3=A9ron?= <georges.duperon@gmail.com>
Date: Sun, 2 Oct 2016 21:25:24 +0200
Subject: [PATCH] Started describing define-fold

---
 traversal.hl.rkt | 97 ++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 97 insertions(+)
 create mode 100644 traversal.hl.rkt

diff --git a/traversal.hl.rkt b/traversal.hl.rkt
new file mode 100644
index 0000000..4daa633
--- /dev/null
+++ b/traversal.hl.rkt
@@ -0,0 +1,97 @@
+#lang hyper-literate typed/racket/base #:no-require-lang #:no-auto-require
+@(require racket/require
+          scribble-enhanced/doc
+          racket/require
+          hyper-literate
+          (subtract-in scribble/struct scribble-enhanced/doc)
+          scribble/decode
+          (for-label racket/format
+                     racket/promise
+                     racket/list
+                     type-expander
+                     (except-in (subtract-in typed/racket/base type-expander)
+                                values)
+                     (only-in racket/base values)
+                     (subtract-in racket/contract typed/racket/base)
+                     phc-toolkit
+                     phc-toolkit/untyped-only
+                     remember))
+@(unless-preexpanding
+  (require (for-label (submod ".."))))
+@doc-lib-setup
+
+@title{Parametric replacement of parts of data structures, identified by their
+ type}
+
+@section{Introduction}
+
+This utility allows functionally updating parts of data structures. The
+@racket[define-fold] macro takes the type of the whole data structure and a
+list of type names associated with their predicate. It locates all literal
+occurrences of those type names within the data structure, and identifies
+those locations as the parts to replace. The type of the whole data structure
+is expressed as a syntactic tree. Within that syntactic tree, only the parts
+which are syntactically equal to one of the types to replace are considered.
+
+As an example, suppose the whole type is
+@racket[(List Foo Number (Listof String))], and @racket[Foo] is defined as:
+
+@racketblock[(define-type Foo (Listof String))]
+
+If @racket[Foo] is given as a type to replace, and its replacement type is
+@racket[(Listof Symbol)], then the type of the result would be:
+
+@racketblock[(List (Listof Symbol) Number (Listof String))]
+
+The second occurrence of @racket[(Listof String)], although semantically
+equivalent to the type to replace, @racket[Foo], will not be altered, as it is
+not expressed syntactically using the @racket[Foo] identifier.
+
+@defform[(define-fold name whole-type
+           [type-to-replaceᵢ predicateᵢ] ...)]{
+ The @racket[define-fold] macro takes the type of the whole data structure, and
+ a list of types to replace, each associated with a predicate for that type. It
+ defines @racket[_name] as a macro, which behaves as follows:
+
+ @(make-blockquote
+   "leftindent"
+   (flow-paragraphs
+    (decode-flow
+     (splice-run
+      @defform[#:link-target? #f (_name function-name
+                                        new-whole-type-name
+                                        accumulator-type
+                                        [new-typeᵢ update-functionᵢ] ...)]{
+  Each @racket[update-functionᵢ] must have the type:
+
+  @racketblock[(→ accumulator-type
+                  type-to-replaceᵢ
+                  (values accumulator-type new-typeᵢ))]
+                                                                           
+  This macro defines @racket[new-whole-type-name] as a type similar to
+  @racket[whole-type], but with each syntactic occurrence of a
+  @racket[type-to-replaceᵢ] replaced with the corresponding @racket[new-typeᵢ].
+
+  This macro also defines @racket[function-name] as a function with the type:
+
+  @racketblock[(→ accumulator-type
+                  whole-type
+                  (values accumulator-type new-whole-type-name))]
+
+  The @racket[function-name] replaces all values in the whole data structure
+  which are present in locations corresponding to a @racket[type-to-replaceᵢ].
+  Each value is passed as an argument to the corresponding
+  @racket[update-functionᵢ], and the result is used as a replacement.
+
+  An accumulator value, with the type @racket[accumulator-type], is threaded
+  through all calls to all @racket[update-functionᵢ], so that the update
+  functions can share state in a functional way.}))))}
+
+
+@chunk[<define-fold>
+       ]
+
+@section{Putting it all together}
+
+@chunk[<*>
+       ]
\ No newline at end of file