ob-core.el: Add ability to use closures as default header arguments

* lisp/ob-core.el (org-babel-default-header-args): Document ability to
use functions.
(eval-default-headers): New function to generate default header
arguments, which adds the ability to evaluate function arguments at
runtime.
(org-babel-get-src-block-info): Use new header argument evaluate
function when retreiving src block info.

* lisp/ob-exp.el (org-babel-exp-src-block): Must use new
eval-default-headers when exporting as well.

The closures are evaluated at runtime.

lisp/ob-core.el

@@ -472,7 +472,23 @@ For the format of SAFE-LIST, see `org-babel-safe-header-args'."
 (defvar org-babel-default-header-args
   '((:session . "none") (:results . "replace") (:exports . "code")
     (:cache . "no") (:noweb . "no") (:hlines . "no") (:tangle . "no"))
-  "Default arguments to use when evaluating a source block.")
+  "Default arguments to use when evaluating a source block.
+
+This is a list in which each element is an alist.  Each key
+corresponds to a header argument, and each value to that header's
+value.  The value can either be a string or a closure that
+evaluates to a string at runtime.  For instance, imagine you'd
+like to set the file name output of a latex source block to a
+sha1 of its contents.  We could achieve this with:
+
+(defun org-src-sha ()
+  (let ((elem (org-element-at-point)))
+    (concat (sha1 (org-element-property :value elem)) \".svg\")))
+
+(setq org-babel-default-header-args:latex
+      `((:results . \"file link replace\")
+        (:file . (lambda () (org-src-sha)))))")
+
 (put 'org-babel-default-header-args 'safe-local-variable
      (org-babel-header-args-safe-fn org-babel-safe-header-args))
 
@@ -583,6 +599,18 @@ the outer-most code block.")
 
 (defvar *this*)
 
+(defun eval-default-headers (headers)
+  "Compute default header list set with HEADERS.
+
+  Evaluate all default header arguments set to functions prior to
+  returning the list of header arguments."
+  (let ((lst nil))
+    (dolist (elem (eval headers t))
+      (if (listp (cdr elem))
+          (push `(,(car elem) . ,(funcall (cdr elem))) lst)
+        (push elem lst)))
+    lst))
+
 (defun org-babel-get-src-block-info (&optional light datum)
   "Extract information from a source block or inline source block.
 
@@ -614,7 +642,7 @@ a list with the following pattern:
 	       (apply #'org-babel-merge-params
 		      (if inline org-babel-default-inline-header-args
 			org-babel-default-header-args)
-		      (and (boundp lang-headers) (eval lang-headers t))
+		      (and (boundp lang-headers) (eval-default-headers lang-headers))
 		      (append
 		       ;; If DATUM is provided, make sure we get node
 		       ;; properties applicable to its location within

lisp/ob-exp.el

@@ -104,7 +104,7 @@ Assume point is at block opening line."
 		       (apply #'org-babel-merge-params
 			      org-babel-default-header-args
 			      (and (boundp lang-headers)
-				   (symbol-value lang-headers))
+				   (eval-default-headers lang-headers))
 			      (append (org-babel-params-from-properties lang)
 				      (list raw-params)))))))
 	  (setf hash (org-babel-sha1-hash info :export)))