From 770b738fe62112fc16c5213bd5769a8a54074fc7 Mon Sep 17 00:00:00 2001
From: Arun Isaac
Date: Fri, 1 Dec 2023 22:34:03 +0000
Subject: ccwl: Implement identity construct.

* ccwl/ccwl.scm (collect-steps): Implement identity construct.
* doc/ccwl.skb (Cookbook)[The identity construct]: New section.
* doc/identity-construct.scm: New file.
---
 ccwl/ccwl.scm              |  4 +++-
 doc/ccwl.skb               |  9 ++++++++-
 doc/identity-construct.scm | 14 ++++++++++++++
 3 files changed, 25 insertions(+), 2 deletions(-)
 create mode 100644 doc/identity-construct.scm

diff --git a/ccwl/ccwl.scm b/ccwl/ccwl.scm
index d00609d..bbc5de1 100644
--- a/ccwl/ccwl.scm
+++ b/ccwl/ccwl.scm
@@ -625,7 +625,7 @@ supplied input keys."
 output keys and a list of steps. INPUT-KEYS is a list of supplied
 input keys. Keys are represented by <key> objects, and steps are
 represented by <step> objects."
-  (syntax-case x (pipe tee rename scatter scatter-cross scatter-nested-cross)
+  (syntax-case x (pipe tee identity rename scatter scatter-cross scatter-nested-cross)
     ;; pipe
     ((pipe expressions ...)
      (foldn (lambda (expression input-keys steps)
@@ -638,6 +638,8 @@ represented by <step> objects."
     ((tee expressions ...)
      (append-mapn (cut collect-steps <> input-keys)
                   #'(expressions ...)))
+    ((identity)
+     (values input-keys (list)))
     ;; rename keys (base case)
     ((rename new-key old-key)
      (begin
diff --git a/doc/ccwl.skb b/doc/ccwl.skb
index 726b405..cb78fd5 100644
--- a/doc/ccwl.skb
+++ b/doc/ccwl.skb
@@ -382,7 +382,14 @@ the external CWL workflow.]
          (scheme-source "doc/external-cwl-workflow.scm")
          [,(file "echo.cwl") is defined as]
          ;; TODO: Syntax highlight doc/external-cwl-workflow.cwl.
-         (prog :line #f (source :file "doc/echo.cwl")))))
+         (prog :line #f (source :file "doc/echo.cwl"))))
+    (section :title [The ,(code [identity]) construct]
+             :ident "identity-construct"
+      (p [Sometimes, it is helpful for a step to simply copy all input
+keys forward to the output. This is what the ,(code [identity])
+construct is for. An example follows.]
+         (scheme-source "doc/identity-construct.scm")
+         (image :file "doc/identity-construct.png"))))
 
   (chapter :title [Contributing]
            :ident "chapter-contributing"
diff --git a/doc/identity-construct.scm b/doc/identity-construct.scm
new file mode 100644
index 0000000..626568f
--- /dev/null
+++ b/doc/identity-construct.scm
@@ -0,0 +1,14 @@
+(define print-message
+  (command #:inputs (message #:type string)
+           #:run "echo" message
+           #:outputs (printed-message #:type stdout)))
+
+(define print-file
+  (command #:inputs (file #:type File)
+           #:run "cat" file
+           #:outputs (printed-file #:type stdout)))
+
+(workflow ((message #:type string))
+  (pipe (print-message #:message message)
+        (tee (print-file #:file printed-message)
+             (identity))))
-- 
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