#5732: preserve compact | pipe when target formation floats away

eo-printer/src/main/java/org/eolang/printer/Xmir.java

@@ -50,6 +50,7 @@ public final class Xmir implements XML {
                 new StClasspath("/org/eolang/printer/print/dataized-to-const.xsl"),
                 new StClasspath("/org/eolang/printer/print/unnecessary-as.xsl"),
                 new StClasspath("/org/eolang/printer/print/merge-monikers.xsl"),
+                new StClasspath("/org/eolang/printer/print/renest-pipe-monikers.xsl"),
                 new StClasspath("/org/eolang/printer/print/to-eo-tree.xsl")
             )
         )

eo-printer/src/main/resources/org/eolang/printer/print/renest-pipe-monikers.xsl

@@ -0,0 +1,112 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!--
+* SPDX-FileCopyrightText: Copyright (c) 2016-2026 Objectionary.com
+* SPDX-License-Identifier: MIT
+-->
+<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" xmlns:eo="https://www.eolang.org" xmlns:xs="http://www.w3.org/2001/XMLSchema" exclude-result-prefixes="xs" id="renest-pipe-monikers" version="2.0">
+  <!--
+  Re-nests a pipe target formation back above its pipe use-site, restoring
+  the compact `| args` continuation line (§3.14). A `| args` line applies
+  its same-indent predecessor; the parser rewrites it into an application
+  whose `@base` points at that predecessor's name and keeps a `@pipe`
+  marker (#5684). When the predecessor formation sits inside an argument
+  block (its parent is itself an application), `wrap-applications` /
+  `vars-float-up` hoist that formation up to the enclosing abstract object
+  and drop it from the argument slot (#5526), leaving the pipe application
+  as the block's sole remaining member. The predecessor has then floated
+  away from the pipe, so `to-eo-tree`'s adjacency guard fails and the line
+  prints as an ordinary application instead of a `|` — the compact author
+  spelling is lost on the first format.
+
+  This pass mirrors `merge-monikers`: for a formation attribute whose only
+  reference is a single `@pipe` application that no longer sits directly
+  after it, it copies the formation back in front of that pipe use-site and
+  drops the floated original. The pipe then reads its predecessor from the
+  immediately preceding sibling, so the adjacency guard at
+  `to-eo-tree.xsl` fires and the `|` head is emitted. Both spellings parse
+  to identical XMIR, so re-nesting is safe.
+
+  Only a formation with exactly one reference is re-nested, and that
+  reference must be the pipe use itself. This is the same single-host
+  condition `merge-monikers` uses (#5706): a second reference would fall
+  out of scope once the formation moved into the pipe's argument block, and
+  the choice of a use-site would not survive the print/parse round trip.
+  A formation already adjacent to its pipe use (never floated, or floated
+  but still landing next to it) is left untouched.
+  -->
+  <xsl:import href="/org/eolang/parser/_funcs.xsl"/>
+  <xsl:output encoding="UTF-8" method="xml"/>
+  <!--
+  The leading name a reference `@base` resolves to: the first segment after
+  an optional `ξ.` root. `ξ.hello` and a bare `hello` both resolve to
+  `hello`; `ξ.hello.tail` or `ξ.ρ.hello` resolve to their own first
+  segment and so never collide with the target name.
+  -->
+  <xsl:function name="eo:ref-head" as="xs:string">
+    <xsl:param name="base" as="xs:string"/>
+    <xsl:variable name="tail" select="if (starts-with($base, concat($eo:xi, '.'))) then substring-after($base, concat($eo:xi, '.')) else $base"/>
+    <xsl:sequence select="tokenize($tail, '\.')[1]"/>
+  </xsl:function>
+  <!--
+  Whether `$attr` is a formation attribute eligible to host a re-nested
+  pipe: named, abstract (a formation, so it is what a pipe predecessor
+  becomes after floating up), owned by an abstract object, and neither `φ`
+  nor a test.
+  -->
+  <xsl:function name="eo:pipe-binding" as="xs:boolean">
+    <xsl:param name="attr" as="element()"/>
+    <xsl:sequence select="exists($attr/@name) and eo:abstract($attr) and eo:abstract($attr/..) and $attr/@name != $eo:phi and not(eo:test-attr($attr))"/>
+  </xsl:function>
+  <!--
+  The references to the binding `$attr`'s name reachable through no
+  intervening formation (so the name still binds to the same formation) and
+  not sitting inside the binding itself (a recursive self-reference).
+  -->
+  <xsl:function name="eo:pipe-refs" as="element()*">
+    <xsl:param name="attr" as="element()"/>
+    <xsl:variable name="owner" select="$attr/.."/>
+    <xsl:sequence select="$owner//o[exists(@base) and eo:ref-head(@base) = $attr/@name and (ancestor::o[eo:abstract(.)][1] is $owner) and not(ancestor::o[. is $attr])]"/>
+  </xsl:function>
+  <!--
+  Whether `$attr` is a pipe binding that has floated away from its use-site:
+  its single reference is a `@pipe` application that is not already its
+  immediate following sibling.
+  -->
+  <xsl:function name="eo:pipe-floated" as="xs:boolean">
+    <xsl:param name="attr" as="element()"/>
+    <xsl:variable name="refs" select="eo:pipe-refs($attr)"/>
+    <xsl:sequence select="eo:pipe-binding($attr) and count($refs) = 1 and exists($refs[1]/@pipe) and not($refs[1]/preceding-sibling::o[1] is $attr)"/>
+  </xsl:function>
+  <!--
+  The floated binding that a reference `$ref` should be re-nested in front
+  of, or the empty sequence when `$ref` hosts none.
+  -->
+  <xsl:function name="eo:pipe-host" as="element()*">
+    <xsl:param name="ref" as="element()"/>
+    <xsl:variable name="owner" select="$ref/ancestor::o[eo:abstract(.)][1]"/>
+    <xsl:sequence select="$owner/o[eo:pipe-floated(.) and eo:pipe-refs(.)[1] is $ref][1]"/>
+  </xsl:function>
+  <!--
+  Emit the re-nested binding immediately before its pipe use-site, then the
+  pipe application itself.
+  -->
+  <xsl:template match="o[exists(eo:pipe-host(.))]" priority="1">
+    <xsl:variable name="binding" select="eo:pipe-host(.)"/>
+    <xsl:element name="o">
+      <xsl:apply-templates select="$binding/@*"/>
+      <xsl:apply-templates select="$binding/node()"/>
+    </xsl:element>
+    <xsl:copy>
+      <xsl:apply-templates select="@*|node()"/>
+    </xsl:copy>
+  </xsl:template>
+  <!--
+  Drop the floated binding once it has been re-nested onto its pipe.
+  -->
+  <xsl:template match="o[eo:pipe-floated(.)]" priority="1"/>
+  <xsl:template match="node()|@*">
+    <xsl:copy>
+      <xsl:apply-templates select="node()|@*"/>
+    </xsl:copy>
+  </xsl:template>
+</xsl:stylesheet>

eo-printer/src/main/resources/org/eolang/printer/print/to-eo-tree.xsl

@@ -237,9 +237,11 @@
   restore the compact "|" head only when that predecessor is still the
   immediately preceding sibling, so the emitted pipe has a valid target
   directly above it — the base then reads "ξ.<name>" (or the bare
-  "<name>" if reference resolution has not rooted it). When the
-  predecessor has floated away (#5526) the guard fails and the node
-  prints as an ordinary application, which round-trips just as safely.
+  "<name>" if reference resolution has not rooted it). A predecessor
+  that floated away (#5526) is re-nested back above its single pipe use by
+  "renest-pipe-monikers" (#5732), so the guard fires here too; only a
+  formation used by more than one pipe stays floated and prints as an
+  ordinary application, which round-trips just as safely.
   -->
   <xsl:template match="o[@pipe and (@base = concat($eo:xi, '.', preceding-sibling::o[1]/@name) or @base = preceding-sibling::o[1]/@name)]" mode="head" priority="2">
     <xsl:text>|</xsl:text>

eo-printer/src/test/resources/org/eolang/printer/print-packs/yaml/pipe-floated-recursive-helper.yaml

@@ -0,0 +1,31 @@
+# SPDX-FileCopyrightText: Copyright (c) 2016-2026 Objectionary.com
+# SPDX-License-Identifier: MIT
+---
+# yamllint disable rule:line-length
+# A recursive helper written inline with `|` (the #5732 tuple/front.eo case):
+# the `[t] > rechead` formation floats up to become an attribute of `front`
+# while its `| list` pipe use stays nested inside the `if.` expression. The
+# printer must re-nest the formation above the pipe so the compact `|`
+# spelling survives instead of degrading into a bare `rechead list`.
+penalties:
+  INDENT: 3
+  BRACKET: 7
+  EXCESS: 1
+  WIDTH: 80
+  STEP: 2
+  SPACE: 7
+origin: |
+  [] > front
+    if. > @
+      idx.eq 0
+      empty
+      [t] > rechead
+        rechead t.tail > @
+      | list
+printed: |
+  [] > front
+    if. > @
+      idx.eq 0
+      empty
+      ^.rechead t.tail > [t] > rechead
+      | list

eo-printer/src/test/resources/org/eolang/printer/print-packs/yaml/pipe-in-argument-block.yaml

@@ -18,5 +18,5 @@ origin: |
 printed: |
   [] > foo
     bar > @
-      hello 5
-    [] > hello
+      [] > hello
+      | 5