Changes between Version 11 and Version 12 of Records/DeclaredOverloadedRecordFields

Timestamp:

02/21/12 15:33:47 (15 months ago)

Author:

guest

Comment:

--

Records/DeclaredOverloadedRecordFields

@@ -54 +54 @@
 
 We need a way to declare that a name is available as an overloadable field name (roughly speaking, a class/method definition), proposed syntax:
+
+'''Option One: new `fieldLabel` style of declaration:'''
 {{{
     fieldLabel customer_id :: r -> Int
 }}}
-(The `r{ ... }` is added by the desugarer.)
+    (The `r{ ... }` is added by the desugarer.)
+
+'''Option Two: explicit record constraint on the function:'''
+{{{
+    customer_id :: r{ customer_id :: Int} => r -> Int          -- field name same as the declared function
+}}}
+    (See discussion at [wiki:Records/DeclaredOverloadedRecordFields/COmpareSORF#TheStringtypeparametertoHasandScopecontrol Wild afterthought].)
 
 The `-> Int` means the field's domain (type) is `Int` -- it's just a type.
 We might also want to constrain the record -- for example to be sure it is savable to persistent storage:
 {{{
-    fieldLabel unitPrice :: (Save r, Num t) => r -> t
-}}}
-(Again the `r{ ... }` gets added as a further constraint.)
+    fieldLabel unitPrice :: (Save r, Num t) => r -> t    -- again the `r{ ... }` gets added as a further constraint
+-- or
+    unitPrice :: (r{ unitPrice :: t}, Save r, Num t) => r -> t     -- again repeated field name
+}}}
+
 
 Now we can use the field in a record, and that in effect declares an instance for the field/record. All these definitions are in the same module:
@@ -76 +86 @@
     data Customer_Order = Cust_Order { customer_id :: Int, ... }
 }}}
-Then a field selection expression like:
+
+=== Field Selection ===
+
+With those records declared, a field selection expression like:
     `... (customer_id r) ...`          -- H98 style field application
 uses familiar type instance resolution to figure out from record type `r` how to extract the `customer_id`.
@@ -90 +103 @@
 The type inferred would be:
 {{{
-    fullName :: r{ firstName, lastName :: String} => r -> String
+    fullName :: r{ firstName, lastName :: String} => r -> String               -- could declare this for yourself
+                                                 -- note this is __not__ like a field label decl (Option Two)
+                                                 -- because the function's name is different to the field(s)
 }}}
 which is eliding:
@@ -97 +112 @@
                  => r -> String
 }}}
-And if you think that's very close to the type of a field selector function, you'd be right. Here's some more examples of pseudo- or 'virtual' fields, using dot notation:
+And if you think that's very close to the type of a field selector function, you'd be right. Here's some more examples of field selection using '''pseudo-''' or''' 'virtual' '''fields, with dot notation:
 {{{
     customer.fullName