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--- codesnippets:typedefaulting [2021/03/17 08:07] – created f2b216
+++ codesnippets:typedefaulting [2025/10/08 00:48] (current) – external edit 127.0.0.1
@@ Line 1: / Line 1: @@
 ====== Type defaulting ======
-<code Haskell>
+Stil not clear how this works:
+  * <code Haskell>
+module Main where
+main :: IO ()
+main = do
+    print $ negate 4
+</code>
+Output:
+  * <code>
+-4
+</code>
+...or this:
+  * <code Haskell>
+module Main where
+main :: IO ()
+main = do
+    print $ sin 3
+</code>
+Output:
+  * <code>
+.1411200080598672
+</code>
+Despite, lentgthy discussions here: [[https://www.reddit.com/r/haskell/comments/1r3w3w/implicit_type_conversion_for_value_constants/]]
+And despite this: [[https://www.haskell.org/onlinereport/decls.html#sect4.3.4]]
+And despite this: [[https://kseo.github.io/posts/2017-01-04-type-defaulting-in-haskell.html]]
+...which states: "Haskell default rule can be summarized as:"
+  * <code Haskell>
 default Num Integer
 default Real Integer
@@ Line 12: / Line 52: @@
 </code>
+Latest reserach in GHC.Num  shows:
+  * <code Haskell>
+module GHC.Num (module GHC.Num, module GHC.Integer, module GHC.Natural) where
+#include "MachDeps.h"
+import GHC.Base
+import GHC.Integer
+import GHC.Natural
+infixl 7  *
+infixl 6  +, -
+default ()              -- Double isn't available yet,
+                        -- and we shouldn't be using defaults anyway
+</code>
+Even more puzzling, this here does not compile:
+  * <code Haskell>
+module Main where
+default ()
+main :: IO ()
+main = do
+    print $ sin 3
+</code>
+Compiler error:
+  * <code>
+app\Main.hs:7:5: error:
+    * Ambiguous type variable `a0' arising from a use of `print'
+      prevents the constraint `(Show a0)' from being solved.
+      Probable fix: use a type annotation to specify what `a0' should be.
+      These potential instances exist:
+        instance Show Ordering -- Defined in `GHC.Show'
+        instance Show Integer -- Defined in `GHC.Show'
+        instance Show a => Show (Maybe a) -- Defined in `GHC.Show'
+        ...plus 22 others
+        ...plus 28 instances involving out-of-scope types
+        (use -fprint-potential-instances to see them all)
+    * In a stmt of a 'do' block: print $ sin 3
+      In the expression: do print $ sin 3
+      In an equation for `main': main = do print $ sin 3
+  |
+|     print $ sin 3
+  |     ^^^^^^^^^^^^^
+</code>
+It will work again, like this:
+  * <code Haskell>
+module Main where
+default (Float)
+main :: IO ()
+main = do
+    print $ sin 3
+</code>
+Output:
+  * <code>
+.14112
+</code>
+===== ✎ =====
+~~DISCUSSION~~