Document GHC-77539 (#518)

Fixes #432.

Author: david-christiansen

message-index/messages/GHC-77539/index.md

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+---
+title: Illegal tuple constraint
+summary: A tuple of constraints was used without enabling the ConstraintKinds extension
+extension: ConstraintKinds
+introduced: 9.6.1
+severity: error
+---
+
+Constraints are the part of a signature that defines the type classes that must be implemented for the types used to instantiate the type variables, found to the left of the double arrow (`=>`).
+In [Haskell 2010](https://www.haskell.org/onlinereport/haskell2010/haskellch4.html#x10-630004.1), type class constraints are either:
+
+ * a single constraint that consists of a named class applied to arguments, or
+
+ * multiple constraints in parentheses, separated by commas.
+
+This strict syntax is necessary because type classes do not themselves form types in Haskell 2010.
+This syntax does not admit nested parentheses or tuples.
+
+With GHC's [`ConstraintKinds` extension](https://ghc.gitlab.haskell.org/ghc/doc/users_guide/exts/constraint_kind.html), type classes form types that have kind `Constraint`, and instead of checking for a specific syntactic form, the type checker ensures that the constraint section of a signature has kind `Constraint`.
+Because tuples of types that have kind `Constraint` themselves have kind `Constraint`, nested tuples are allowed.
+This is especially convenient when defining type synonyms that stand for tuples of constraints.

message-index/messages/GHC-77539/tuple-constraint/after/TupleConstraint.hs

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+{-# LANGUAGE ConstraintKinds #-}
+module TupleConstraint where
+
+addFromString :: ((Read a, Show a), Num a) => String -> a -> String
+addFromString x y = show (read x + y)

message-index/messages/GHC-77539/tuple-constraint/before/TupleConstraint.hs

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+{-# LANGUAGE Haskell2010 #-}
+module TupleConstraint where
+
+addFromString :: ((Read a, Show a), Num a) => String -> a -> String
+addFromString x y = show (read x + y)

message-index/messages/GHC-77539/tuple-constraint/index.md

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+---
+title: Use of a tuple constraint
+---
+
+## Message
+```
+TupleConstraint.hs:4:18: error: [GHC-77539]
+    • Illegal tuple constraint: (Read a, Show a)
+    • In the type signature:
+        addFromString :: ((Read a, Show a), Num a) => String -> a -> String
+    Suggested fix: Perhaps you intended to use ConstraintKinds
+  |
+4 | addFromString :: ((Read a, Show a), Num a) => String -> a -> String
+  |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+```
+
+## Explanation
+
+This file is written in Haskell 2010, which does not have `ConstraintKinds`.
+This means that a syntactic check is used to ensure that type class constraints form a single-level tuple, and this file does not satisfy that requirement.
+Either de-nesting the tuple or enabling `ConstraintKinds` fixes the issue.