debug/1 (#2710)

* debug/1

This def ensures the output of debug(m1,m2) is kept together.

Closes #2709 #2111 #2112

3 files changed, 75 insertions, 7 deletions

diff --git a/docs/content/manual/manual.yml b/docs/content/manual/manual.yml
index 9dbd1b19..24db2daf 100644
--- a/docs/content/manual/manual.yml
+++ b/docs/content/manual/manual.yml
@@ -3075,13 +3075,32 @@ sections:
           to invoke jq with the -n command-line option, otherwise
           the first entity will be lost.
 
-      - title: "`debug`"
+      - title: "`debug`, `debug(msgs)`"
         body: |
 
-          Causes a debug message based on the input value to be
-          produced.  The jq executable wraps the input value with
-          `["DEBUG:", <input-value>]` and prints that and a newline on
-          stderr, compactly.  This may change in the future.
+          These two filters are like `.` but have as a side-effect the
+          production of one or more messages on stderr.
+
+          The message produced by the `debug` filter has the form
+
+              ["DEBUG:",<input-value>]
+
+          where `<input-value>` is a compact rendition of the input
+          value.  This format may change in the future.
+
+          The `debug(msgs)` filter is defined as `(msgs | debug | empty), .`
+          thus allowing great flexibility in the content of the message,
+          while also allowing multi-line debugging statements to be created.
+
+          For example, the expression:
+
+              1 as $x | 2 | debug("Entering function foo with $x == \($x)", .) | (.+1)
+
+          would produce the value 3 but with the following two lines
+          being written to stderr:
+
+              ["DEBUG:","Entering function foo with $x == 1"]
+              ["DEBUG:",2]
 
       - title: "`stderr`"
         body: |
diff --git a/jq.1.prebuilt b/jq.1.prebuilt
index 37f40341..d4979985 100644
--- a/jq.1.prebuilt
+++ b/jq.1.prebuilt
@@ -3353,8 +3353,54 @@ Outputs all remaining inputs, one by one\.
 .P
 This is primarily useful for reductions over a program\'s inputs\. Note that when using \fBinputs\fR it is generally necessary to invoke jq with the \-n command\-line option, otherwise the first entity will be lost\.
 .
-.SS "debug"
-Causes a debug message based on the input value to be produced\. The jq executable wraps the input value with \fB["DEBUG:", <input\-value>]\fR and prints that and a newline on stderr, compactly\. This may change in the future\.
+.SS "debug, debug(msgs)"
+These two filters are like \fB\.\fR but have as a side\-effect the production of one or more messages on stderr\.
+.
+.P
+The message produced by the \fBdebug\fR filter has the form
+.
+.IP "" 4
+.
+.nf
+
+["DEBUG:",<input\-value>]
+.
+.fi
+.
+.IP "" 0
+.
+.P
+where \fB<input\-value>\fR is a compact rendition of the input value\. This format may change in the future\.
+.
+.P
+The \fBdebug(msgs)\fR filter is defined as \fB(msgs | debug | empty), \.\fR thus allowing great flexibility in the content of the message, while also allowing multi\-line debugging statements to be created\.
+.
+.P
+For example, the expression:
+.
+.IP "" 4
+.
+.nf
+
+1 as $x | 2 | debug("Entering function foo with $x == \e($x)", \.) | (\.+1)
+.
+.fi
+.
+.IP "" 0
+.
+.P
+would produce the value 3 but with the following two lines being written to stderr:
+.
+.IP "" 4
+.
+.nf
+
+["DEBUG:","Entering function foo with $x == 1"]
+["DEBUG:",2]
+.
+.fi
+.
+.IP "" 0
 .
 .SS "stderr"
 Prints its input in raw and compact mode to stderr with no additional decoration, not even a newline\.
diff --git a/src/builtin.jq b/src/builtin.jq
index 146a64a3..a13d7845 100644
--- a/src/builtin.jq
+++ b/src/builtin.jq
@@ -266,6 +266,9 @@ def pick(pathexps):
   | reduce path(pathexps) as $a (null;
       setpath($a; $in|getpath($a)) );
 
+# ensure the output of debug(m1,m2) is kept together:
+def debug(msgs): (msgs | debug | empty), .;
+
 # SQL-ish operators here:
 def INDEX(stream; idx_expr):
   reduce stream as $row ({}; .[$row|idx_expr|tostring] = $row);