Merge commit '95aeb2ebae' as 'third_party/lisp/alexandria'

third_party/lisp/alexandria/binding.lisp

@@ -0,0 +1,90 @@
+(in-package :alexandria)
+
+(defmacro if-let (bindings &body (then-form &optional else-form))
+    "Creates new variable bindings, and conditionally executes either
+THEN-FORM or ELSE-FORM. ELSE-FORM defaults to NIL.
+
+BINDINGS must be either single binding of the form:
+
+ (variable initial-form)
+
+or a list of bindings of the form:
+
+ ((variable-1 initial-form-1)
+  (variable-2 initial-form-2)
+  ...
+  (variable-n initial-form-n))
+
+All initial-forms are executed sequentially in the specified order. Then all
+the variables are bound to the corresponding values.
+
+If all variables were bound to true values, the THEN-FORM is executed with the
+bindings in effect, otherwise the ELSE-FORM is executed with the bindings in
+effect."
+    (let* ((binding-list (if (and (consp bindings) (symbolp (car bindings)))
+                             (list bindings)
+                             bindings))
+         (variables (mapcar #'car binding-list)))
+    `(let ,binding-list
+       (if (and ,@variables)
+           ,then-form
+           ,else-form))))
+
+(defmacro when-let (bindings &body forms)
+    "Creates new variable bindings, and conditionally executes FORMS.
+
+BINDINGS must be either single binding of the form:
+
+ (variable initial-form)
+
+or a list of bindings of the form:
+
+ ((variable-1 initial-form-1)
+  (variable-2 initial-form-2)
+  ...
+  (variable-n initial-form-n))
+
+All initial-forms are executed sequentially in the specified order. Then all
+the variables are bound to the corresponding values.
+
+If all variables were bound to true values, then FORMS are executed as an
+implicit PROGN."
+  (let* ((binding-list (if (and (consp bindings) (symbolp (car bindings)))
+                           (list bindings)
+                           bindings))
+         (variables (mapcar #'car binding-list)))
+    `(let ,binding-list
+       (when (and ,@variables)
+         ,@forms))))
+
+(defmacro when-let* (bindings &body body)
+  "Creates new variable bindings, and conditionally executes BODY.
+
+BINDINGS must be either single binding of the form:
+
+ (variable initial-form)
+
+or a list of bindings of the form:
+
+ ((variable-1 initial-form-1)
+  (variable-2 initial-form-2)
+  ...
+  (variable-n initial-form-n))
+
+Each INITIAL-FORM is executed in turn, and the variable bound to the
+corresponding value. INITIAL-FORM expressions can refer to variables
+previously bound by the WHEN-LET*.
+
+Execution of WHEN-LET* stops immediately if any INITIAL-FORM evaluates to NIL.
+If all INITIAL-FORMs evaluate to true, then BODY is executed as an implicit
+PROGN."
+  (let ((binding-list (if (and (consp bindings) (symbolp (car bindings)))
+                          (list bindings)
+                          bindings)))
+    (labels ((bind (bindings body)
+               (if bindings
+                   `(let (,(car bindings))
+                      (when ,(caar bindings)
+                        ,(bind (cdr bindings) body)))
+                   `(progn ,@body))))
+      (bind binding-list body))))