Document GHC-99565 (#468)

Author: BinderDavid

message-index/messages/GHC-99565/index.md

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+---
+title: Multiple default declarations
+summary: Multiple default declarations are not allowed
+severity: error
+introduced: 9.6.1
+---
+
+A numeric literal such as `5` is overloaded in Haskell and has the type `forall a. Num a => a`.
+This allows the programmer to use numeric literals in many contexts without having a separate literal syntax for different numeric types.
+By extension, many numeric functions have a polymorphic type involving the `Num` type class.
+
+A downside of using overloaded arithmetic expressions is that GHC is not always able to non-ambiguously infer a concrete type, such as `Int` or `Double`.
+This is the case, for example, in the expression `show 5`.
+The compiler cannot infer if the programmer means `show (5 :: Int)`, `show (5 :: Double)` or any other numeric type.
+
+Since this case is very common, the Haskell report specifies a default behaviour for overloaded numeric expressions which involve the `Num` typeclass.
+Numeric types are defaulted to `Integer`, and if that is not possible to `Double`.
+
+This defaulting behaviour can be customized with "default declarations".
+A default declaration uses the syntax `default (...)` with a list of comma-separated types.
+These types are tried in the order in which they occur in the default declaration.
+However, every Haskell module may contain at most one such default declaration.
+If a module contains more that one default declaration, then this error is thrown by GHC.

message-index/messages/GHC-99565/multiple-defaults/after/MultipleDefaults.hs

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+module MultipleDefaults where
+
+default (Double, Int)
+
+
+main :: IO ()
+main = print 5

message-index/messages/GHC-99565/multiple-defaults/before/MultipleDefaults.hs

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+module MultipleDefaults where
+
+default (Double)
+default (Int)
+
+main :: IO ()
+main = print 5

message-index/messages/GHC-99565/multiple-defaults/index.md

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+---
+title: Multiple default declarations in single module
+---
+
+In this example the programmer tries to customize the defaulting behaviour for numeric types to allow defaulting to both `Double` and `Int`.
+In order to do this both defaults have to be specified in a comma-separated list instead of two separate default declarations.
+
+# Error Message
+
+```
+messages/GHC-99565/multiple-defaults/before/MultipleDefaults.hs:4:1: error:
+    Multiple default declarations
+      here was another default declaration messages/GHC-99565/multiple-defaults/before/MultipleDefaults.hs:3:1-16
+  |
+4 | default (Double)
+  | ^^^^^^^^^^^^^^^^
+```