From 95a7fb87f8e72a80675d99487eb1f554aedd1012 Mon Sep 17 00:00:00 2001
From: AlexKnauth <alexander@knauth.org>
Date: Mon, 2 May 2016 14:29:51 -0400
Subject: [PATCH] =?UTF-8?q?add=20~=3F=E2=88=80=20for=20matching=20possibly?=
 =?UTF-8?q?=20polymorphic=20types?=
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---
 tapl/mlish.rkt | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/tapl/mlish.rkt b/tapl/mlish.rkt
index b103a17..10b93e7 100644
--- a/tapl/mlish.rkt
+++ b/tapl/mlish.rkt
@@ -31,8 +31,19 @@
 ;; - pattern matching
 ;; - (local) type inference
 
-  ;; type inference constraint solving
 (begin-for-syntax 
+  ;; matching possibly polymorphic types
+  (define-syntax ~?∀
+    (pattern-expander
+     (lambda (stx)
+       (syntax-case stx ()
+         [(?∀ vars-pat body-pat)
+          #'(~or (~∀ vars-pat body-pat)
+                 (~and (~not (~∀ _ _))
+                       (~parse vars-pat #'())
+                       body-pat))]))))
+
+  ;; type inference constraint solving
   (define (compute-constraint τ1-τ2)
     (syntax-parse τ1-τ2
       [(X:id τ) #'((X τ))]