atsapura · January 4, 2025 05:27 · Jun 6, 2019 · Jun 6, 2019 · Jun 3, 2019
diff --git a/Srtp and Records.fs b/Srtp and Records.fs
@@ -35,12 +35,16 @@ Active patterns are not required for this, but they do make code much easier to
     let inline printNameAndId (HasName n & HasEntityId id) = sprintf "name %s id %s" (n()) (id())
 
 // Watching it in action:
+    let test1 = { EntityId = "123" }
     let test2 = {Test2.EntityId = "123"; Name = "123"; Length = 2}
 
 // Works both with Test and Test2 types since both have `EntityId` field
-    let a = entityId { EntityId = "123" }
+    let a = entityId test1
     let b = entityId test2
 
     let c = name test2
 
     let d = printNameAndId test2
+
+// *Compile* error when using type without required fields:
+    let error = printNameAndId test1
diff --git a/Srtp and Records.fs b/Srtp and Records.fs
@@ -1,4 +1,13 @@
 
+(*
+WHAT'S GOING ON HERE?!
+
+Sometimes you don't care about a particular type, you're interested in one field only, let's say `EntityId`.
+Instead of using interface (which isn't even possible if don't own a type),
+we can do structural typing in F# using SRTP and Active Patterns.
+Active patterns are not required for this, but they do make code much easier to use.
+*)
+// So we have 2 types with field `EntityId: string`:
     type Test =
         { EntityId: string }
 
@@ -7,18 +16,31 @@
           Name: string 
           Length: int }
 
+// First we define active pattern for detecting
+// whether this type has this field or not:
     let inline (|HasEntityId|) x =
-        fun () -> (^a : (member EntityId: string) x)
+        fun () -> (^a : (member EntityId: string) x) // as you can see, SRTP syntax is hardly comfortable to use
 
+// Then we define function for retrieving this field
     let inline entityId (HasEntityId f) = f()
 
+// Another AP, this time for detecting `Name: string`:
     let inline (|HasName|) n =
         fun () -> (^a : (member Name: string) n)
 
+// and function for getting the name:
     let inline name (HasName f) = f()
-    let inline test2f (HasName n & HasEntityId id) = sprintf "name %s id %s" (n()) (id())
+
+// Now here's the beauty of it: we can combine them!    
+    let inline printNameAndId (HasName n & HasEntityId id) = sprintf "name %s id %s" (n()) (id())
+
+// Watching it in action:
     let test2 = {Test2.EntityId = "123"; Name = "123"; Length = 2}
+
+// Works both with Test and Test2 types since both have `EntityId` field
     let a = entityId { EntityId = "123" }
     let b = entityId test2
+
     let c = name test2
-    let d = test2f test2
+
+    let d = printNameAndId test2
diff --git a/Srtp and Records.fs b/Srtp and Records.fs
@@ -0,0 +1,24 @@
+
+    type Test =
+        { EntityId: string }
+
+    type Test2 =
+        { EntityId: string
+          Name: string 
+          Length: int }
+
+    let inline (|HasEntityId|) x =
+        fun () -> (^a : (member EntityId: string) x)
+
+    let inline entityId (HasEntityId f) = f()
+
+    let inline (|HasName|) n =
+        fun () -> (^a : (member Name: string) n)
+
+    let inline name (HasName f) = f()
+    let inline test2f (HasName n & HasEntityId id) = sprintf "name %s id %s" (n()) (id())
+    let test2 = {Test2.EntityId = "123"; Name = "123"; Length = 2}
+    let a = entityId { EntityId = "123" }
+    let b = entityId test2
+    let c = name test2
+    let d = test2f test2
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