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--- codesnippets:parametrictypeforanintermediatevaluewhencombiningfunctions [2021/04/23 18:40] – created f2b216
+++ codesnippets:parametrictypeforanintermediatevaluewhencombiningfunctions [2025/10/08 00:48] (current) – external edit 127.0.0.1
@@ Line 1: / Line 1: @@
-====== Type for an intermediate value when combining functions ======
+====== Parametric type for an intermediate value when combining functions ======
   * example with the folling problem(s):
-    * function ''buildNRender'' has an ambigious intermediate type
+    * function ''buildNRender'' has an ambigious intermediate parametric type
+    * the type of ''buildNRender'' has no parametric type g involved at all
+      * hence, it does not make sense to have ''buildNRender'' as a member of ''class'' ''Builder''
     * code <code Haskell>
 module Main where
@@ Line 28: / Line 30: @@
     build s = Contains (MyData (read s :: Integer))
     render (Contains (MyData n)) = Oct.toOctets n
-    render (Error _) = []</code>
+    render (Error _) = []
-  * example that solves the problem
+</code>
+    * compiles, with error:<code>
+app\Main.hs:19:5: error:
+    * Could not deduce (Builder g0)
+      from the context: Builder g
+        bound by the type signature for:
+                   buildNRender :: forall g. Builder g => String -> [W.Word8]
+        at app\Main.hs:19:5-39
+      The type variable `g0' is ambiguous
+    * In the ambiguity check for `buildNRender'
+      To defer the ambiguity check to use sites, enable AllowAmbiguousTypes
+      When checking the class method:
+        buildNRender :: forall g. Builder g => String -> [W.Word8]
+      In the class declaration for `Builder'
+   |
+|     buildNRender :: String -> [W.Word8]
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+</code>
+  * example that solves the problem with extensions
     * code <code Haskell>
 {-# LANGUAGE AllowAmbiguousTypes, TypeApplications, ScopedTypeVariables #-}
@@ Line 62: / Line 82: @@
     render (Error _) = []
 </code>
+  * example that solves the problem with proxy passing
+    * NOTE: Proxy has a type parameter that is not used and does not even carry data.
+    * code <code Haskell>
+module Main where
+import qualified Data.Word as W
+import qualified Octetable as Oct
+main :: IO ()
+main =
+    do
+        print (buildNRender (Proxy :: Proxy MyData) "123")
+data MyData = MyData Integer
+data Construction model = Contains model | Error String
+    deriving Show
+class Builder g where
+    build :: String -> (Construction g)
+    render :: (Construction g) -> [W.Word8]
+    buildNRender :: proxy g -> String -> [W.Word8]
+    buildNRender p = render . applyProxy p build
+        where
+            applyProxy :: proxy g -> f (c g) -> f (c g)
+            applyProxy _ = id
+instance Builder MyData where
+    build s = Contains (MyData (read s :: Integer))
+    render (Contains (MyData n)) = Oct.toOctets n
+    render (Error _) = []
+data Proxy a = Proxy
+</code>
+  * example that solves the problem with Tagged
+    * NOTE: Proxy has a type parameter that is not used and does not even carry data.
+    * code <code Haskell>
+module Main where
+import qualified Data.Word as W
+import qualified Octetable as Oct
+import qualified Data.Tagged as Tg
+main :: IO ()
+main =
+    do
+        print ((getTaggedFunc (buildNRender :: Tg.Tagged MyData (String -> [W.Word8]))) "123")
+getTaggedFunc :: Tg.Tagged g (String -> [W.Word8]) -> (String -> [W.Word8])
+getTaggedFunc (Tg.Tagged f) = f
+data MyData = MyData Integer
+data Construction model = Contains model | Error String
+    deriving Show
+class Builder g where
+    build :: String -> (Construction g)
+    render :: (Construction g) -> [W.Word8]
+    buildNRender :: Tg.Tagged g (String -> [W.Word8])
+    buildNRender = (render .) <$> applyTag build
+        where
+            applyTag :: f (c g) -> Tg.Tagged g (f (c g))
+            applyTag = Tg.Tagged
+instance Builder MyData where
+    build s = Contains (MyData (read s :: Integer))
+    render (Contains (MyData n)) = Oct.toOctets n
+    render (Error _) = []
+</code>
+===== ✎ =====
+~~DISCUSSION~~