Fix short-circuiting of --first

* dash.el (--first): Stop looping even if needle is nil.  Extend
docstring.
(-first): Extend docstring.
* dev/examples.el (-first): Extend tests.

* README.md:
* dash.texi: Regenerate docs.

README.md

@@ -1964,16 +1964,16 @@ See also: [`-flatten-n`](#-flatten-n-num-list), [`-table`](#-table-fn-rest-lists
 
 #### -first `(pred list)`
 
-Return the first x in `list` where (`pred` x) is non-nil, else nil.
-
+Return the first item in `list` for which `pred` returns non-nil.
+Return nil if no such element is found.
 To get the first item in the list no questions asked, use `car`.
-
-Alias: `-find`
+Alias: `-find`.
+This function's anaphoric counterpart is `--first`.
 
 ```el
-(-first 'even? '(1 2 3)) ;; => 2
-(-first 'even? '(1 3 5)) ;; => nil
-(-first 'null '(1 3 5)) ;; => nil
+(-first #'natnump '(-1 0 1)) ;; => 0
+(-first #'null '(1 2 3)) ;; => nil
+(--first (> it 2) '(1 2 3)) ;; => 3
 ```
 
 #### -some `(pred list)`

dash.el

@@ -741,20 +741,24 @@ If ELEMENTS is non nil, append these to the list as well."
   (-concat list (list elem) elements))
 
 (defmacro --first (form list)
-  "Anaphoric form of `-first'."
+  "Return the first item in LIST for which FORM evals to non-nil.
+Return nil if no such element is found.
+Each element of LIST in turn is bound to `it' and its index
+within LIST to `it-index' before evaluating FORM.
+This is the anaphoric counterpart to `-first'."
   (declare (debug (form form)))
   (let ((n (make-symbol "needle")))
     `(let (,n)
-       (--each-while ,list (not ,n)
-         (when ,form (setq ,n it)))
+       (--each-while ,list (or (not ,form)
+                               (ignore (setq ,n it))))
        ,n)))
 
 (defun -first (pred list)
-  "Return the first x in LIST where (PRED x) is non-nil, else nil.
-
+  "Return the first item in LIST for which PRED returns non-nil.
+Return nil if no such element is found.
 To get the first item in the list no questions asked, use `car'.
-
-Alias: `-find'"
+Alias: `-find'.
+This function's anaphoric counterpart is `--first'."
   (--first (funcall pred it) list))
 
 (defalias '-find '-first)

dash.texi

@@ -2973,24 +2973,24 @@ See also: @code{-flatten-n} (@pxref{-flatten-n}), @code{-table} (@pxref{-table})
 
 @anchor{-first}
 @defun -first (pred list)
-Return the first x in @var{list} where (@var{pred} x) is non-nil, else nil.
-
+Return the first item in @var{list} for which @var{pred} returns non-nil.
+Return nil if no such element is found.
 To get the first item in the list no questions asked, use @code{car}.
-
-Alias: @code{-find}
+Alias: @code{-find}.
+This function's anaphoric counterpart is @code{--first}.
 
 @example
 @group
-(-first 'even? '(1 2 3))
-    @result{} 2
+(-first #'natnump '(-1 0 1))
+    @result{} 0
 @end group
 @group
-(-first 'even? '(1 3 5))
+(-first #'null '(1 2 3))
     @result{} nil
 @end group
 @group
-(-first 'null '(1 3 5))
-    @result{} nil
+(--first (> it 2) '(1 2 3))
+    @result{} 3
 @end group
 @end example
 @end defun

dev/examples.el

@@ -980,11 +980,30 @@ value rather than consuming a list to produce a single value."
     (-flatten-n 2 (apply '-table 'list (-repeat 3 '(1 2)))) => '((1 1 1) (2 1 1) (1 2 1) (2 2 1) (1 1 2) (2 1 2) (1 2 2) (2 2 2)))
 
   (defexamples -first
-    (-first 'even? '(1 2 3)) => 2
-    (-first 'even? '(1 3 5)) => nil
-    (-first 'null '(1 3 5)) => nil
-    (-first 'null '(1 3 ())) => nil
-    (--first (> it 2) '(1 2 3)) => 3)
+    (-first #'natnump '(-1 0 1)) => 0
+    (-first #'null '(1 2 3)) => nil
+    (--first (> it 2) '(1 2 3)) => 3
+    (let ((c 0)) (--first (setq c (1+ c)) '(nil nil nil)) c) => 1
+    (--first nil '(1 2 3)) => nil
+    (--first nil '(1)) => nil
+    (--first nil '()) => nil
+    (-first #'ignore '(1 2 3)) => nil
+    (-first #'ignore '(1)) => nil
+    (-first #'ignore '()) => nil
+    (--first (not it) '(1 2 nil)) => nil
+    (--first (not it) '(nil 1 2)) => nil
+    (--first (not it) '(nil)) => nil
+    (--first (not it) '()) => nil
+    (-first #'null '(1 2 nil)) => nil
+    (-first #'null '(nil 1 2)) => nil
+    (-first #'null '(nil)) => nil
+    (-first #'null '()) => nil
+    (--first t '(1 2 3)) => 1
+    (--first t '(1)) => 1
+    (--first t '()) => nil
+    (-first #'identity '(1 2 3)) => 1
+    (-first #'identity '(1)) => 1
+    (-first #'identity '()) => nil)
 
   (defexamples -some
     (-some 'even? '(1 2 3)) => t