From 87deddc879ba2342b6f603ce804c664fe669d708 Mon Sep 17 00:00:00 2001
From: Tony Garnock-Jones <tonygarnockjones@gmail.com>
Date: Mon, 19 Nov 2018 16:55:22 +0000
Subject: [PATCH] RecordConstructorInfo for python

---
 implementations/python/preserves/preserves.py | 38 +++++++++++++++++++
 1 file changed, 38 insertions(+)

diff --git a/implementations/python/preserves/preserves.py b/implementations/python/preserves/preserves.py
index a02b238..f434692 100644
--- a/implementations/python/preserves/preserves.py
+++ b/implementations/python/preserves/preserves.py
@@ -75,6 +75,44 @@ class Record(object):
         for f in self.fields:
             encoder.append(f)
 
+    @staticmethod
+    def makeConstructor(labelSymbolText, fieldNames):
+        return Record.makeBasicConstructor(Symbol(labelSymbolText), fieldNames)
+
+    @staticmethod
+    def makeBasicConstructor(label, fieldNames):
+        if type(fieldNames) == str:
+            fieldNames = fieldNames.split()
+        arity = len(fieldNames)
+        def ctor(*fields):
+            if len(fields) != arity:
+                raise Exception("Record: cannot instantiate %r expecting %d fields with %d fields"%(
+                    label,
+                    arity,
+                    len(fields)))
+            return Record(label, fields)
+        ctor.constructorInfo = RecordConstructorInfo(label, arity)
+        ctor.isClassOf = lambda v: \
+                         isinstance(v, Record) and v.key == label and len(v.fields) == arity
+        return ctor
+
+class RecordConstructorInfo(object):
+    def __init__(self, key, arity):
+        self.key = key
+        self.arity = arity
+
+    def __eq__(self, other):
+        return isinstance(other, RecordConstructorInfo) and \
+            (self.key, self.arity) == (other.key, other.arity)
+
+    def __hash__(self):
+        if self.__hash is None:
+            self.__hash = hash((self.key, self.arity))
+        return self.__hash
+
+    def __repr__(self):
+        return str(self.key) + '/' + str(self.arity)
+
 # Blub blub blub
 class ImmutableDict(dict):
     def __init__(self, *args, **kwargs):