From c8c23a1d39ae812f5130a53e5d68726d2dff199a Mon Sep 17 00:00:00 2001
From: Matthew Butterick <mb@mbtype.com>
Date: Mon, 13 Jan 2020 13:45:24 -0800
Subject: [PATCH] preserve imperative changes to metas

---
 pollen/private/main-base.rkt | 13 ++++++++-----
 pollen/private/ts.rktd       |  2 +-
 pollen/test/test-meta.rkt    | 16 +++++++++++++++-
 3 files changed, 24 insertions(+), 7 deletions(-)

diff --git a/pollen/private/main-base.rkt b/pollen/private/main-base.rkt
index e46918a..03851b0 100644
--- a/pollen/private/main-base.rkt
+++ b/pollen/private/main-base.rkt
@@ -53,16 +53,19 @@
             ;; and we want root to see those changes
             (begin0
               (parse xs PARSER-MODE ROOT-ID)
+              ;; pick up any imperative changes to current-metas by tag functions
+              (set! METAS-ID-CALLER (current-metas))
+              ;; restore previous value of metas
               (current-metas prev-metas))) 
           (module METAS-ID racket/base
             (provide METAS-ID)
             (define METAS-ID META-HASH))
           (require POLLEN/TOP (submod "." METAS-ID))
-          (provide ALL-DEFINED-OUT ; implicitly picks up METAS-ID-CALLER
+          (provide ALL-DEFINED-OUT; implicitly picks up METAS-ID-CALLER
                    DOC-ID)
-          (define prev-metas (current-metas))
-          (define METAS-ID-CALLER METAS-ID)
-          (and (current-metas METAS-ID) "") ; because empty strings get stripped, voids don't
           ;; we set current-metas imperatively rather than using `splicing-parameterize`
-          ;; so that we can unset it in the post processor, rather than out here
+          ;; so that we can restore it in the post-processor, rather than out here
+          (define METAS-ID-CALLER METAS-ID) ; grab the new metas
+          (define prev-metas (current-metas)) ; stash the old metas
+          (and (current-metas METAS-ID-CALLER) "") ; because empty strings get stripped, voids don't
           (begin . EXPRS)))])) 
diff --git a/pollen/private/ts.rktd b/pollen/private/ts.rktd
index 24ce079..b34acae 100644
--- a/pollen/private/ts.rktd
+++ b/pollen/private/ts.rktd
@@ -1 +1 @@
-1578891880
+1578951924
diff --git a/pollen/test/test-meta.rkt b/pollen/test/test-meta.rkt
index 81fc633..ce58dba 100644
--- a/pollen/test/test-meta.rkt
+++ b/pollen/test/test-meta.rkt
@@ -7,4 +7,18 @@
   (provide val))
 
 (require rackunit 'metatest)
-(check-equal? val "value")
\ No newline at end of file
+(check-equal? val "value")
+
+;; check that exported metas are a copy of final state of current-metas
+(module metatest2 pollen
+  (define-meta key "value")
+  (define (tag . xs)
+    (current-metas (hash-set (current-metas) 'key "reset"))
+    "")
+  (tag "hello")
+  (define val (hash-ref metas 'key))
+  (provide val))
+
+(require (prefix-in 2: 'metatest2))
+(check-equal? val "value")
+(check-equal? (hash-ref 2:metas 'key) "reset")
\ No newline at end of file