Currently `docs/users-guide/glasgow_exts.rst` is nearly 17kLoC. This is problematic for a number of reasons: * Loading the HTML that it produces is very taxing: Firefox on my laptop requires nearly 5 seconds to completely load the nearly 2 MB of HTML produced in the [8.8.1 documentation](https://downloads.haskell.org/ghc/8.8.1/docs/html/users_guide/glasgow_exts.html). Woe betide you who browse on a mobile device. * Navigation is a challenge: While the heading structure of the page is fairly logical, the TOC on the right side of the page spans 13 vertical screen-fuls on my laptop, rendering it quite useless. * Editing is a nightmare: There is an noticeable lag while editing the ReST source in vim and even orienting oneself in the file is dizzying. * Sphinx struggles to compile it: The file is large enough that we have had to work around performance limitations in Sphinx in the past (#10950) For the above reasons I think we should break up this file. I suggest the following split (referring to the section numbers from the [8.8.1 documentation](https://downloads.haskell.org/ghc/8.8.1/docs/html/users_guide/glasgow_exts.html)). * `glasgow_exts.rst`: Language options (Section 9.1) * `glasgow_exts/values.rst`: (syntax and extensions to the value-level language) * Parallel list comprehensions (9.3.13) * Generalised (SQL-like) list comprehensions (9.3.14) * Monad comprehensions (9.3.15) * New monadic failure desugaring mechanism (9.3.16) * Recursive do-notation (9.3.11) * Applicative do-notation (9.3.12) * Arrow notation (9.28) * Bang patterns and strict Haskell (9.29) * Assertions (9.30) * Static pointers (9.31) * `HasCallStack` (9.38) * Rebindable syntax and the implicit Prelude import (9.3.17) * Post-fix operators (9.3.18) * Tuple sections (9.3.19) * Lambda-case (9.3.20) * Empty case alternatives (9.3.21) * Multi-way-if expressions (9.3.22) * Local fixity declarations (9.3.23) * `glasgow_exts/literals.rst`: * Negative literals (9.3.3) * Fractional-looking integer literals (9.3.4) * Binary integer literals (9.3.5) * Hexadecimal floating-point literals (9.3.6) * Numeric underscores (9.3.7) * `glasgow_exts/syntax.rst`: * Unicode syntax (9.3.1) * Magic hash (9.3.2) * Import and export extensions (9.3.24) * More liberal syntax for function arguments (9.3.25) * Summary of stolen syntax (9.3.26) * `glasgow_exts/types.rst`: * Lexically scoped type variables (9.17) * Bindings and generalisation (9.18) * Extensions to data types and type synonyms (9.4) * Extensions to type signatures (9.16) * Visible type application (9.19) * Datatype promotion (9.10) * Kind polymorphism (9.11) * Levity polymorphism (9.12) * Equality constraints, Coercible, and the kind `Constraint` (9.14) * Arbitrary-rank polymorphism (9.21) * Impredicative polymorphism (9.22) * Roles (9.37) * Type families (9.9) * `glasgow_exts/type_errors.rst`: (not so sure about this; fold into `types.rst`?) * Typed holes (7.23) * Partial type signatures (9.24) * Custom compile-time errors (9.25) * Deferring type errors to runtime (9.26) * `glasgow_exts/records.rst`: Extensions to the record system (9.5) * `glasgow_exts/patterns.rst`: * Pattern guards (9.3.8) * View patterns (9.3.9) * n+k patterns (9.3.10 * Pattern synonyms (9.7) * `COMPLETE` pragmas (9.32.15) * `glasgow_exts/typeclasses.rst`: * Implicit parameters (9.20) ? * `MINIMAL` pragma (9.32.5) * `OVERLAPPING`, `OVERLAPPABLE`, `OVERLAPS`, and `INCOHERENT` pragmas (9.32.17) * `glasgow_exts/deriving.rst`: * Extensions to the “deriving” mechanism (9.6) * Class and instances declarations (9.8) * Quantified constraints (9.15) * `glasgow_exts/template_haskell.rst`: Template Haskell (9.27) * `glasgow_exts/simplification.rst`: Mechanisms to guide simplification * `INLINE` and `NOINLINE` pragmas (9.32.6) * `RULES` pragma (9.32.9) * `SPECIALISE` pragma (9.32.10) * `SPECIALISE instance` pragma (9.32.11) * `UNPACK` pragma (9.32.12) * `NOUNPACK` pragma (9.32.12) * Rewrite rules (9.33) * Special built-in functions (9.34) * `glasgow_exts/misc_pragmas.rst`: Miscellaneous pragmas * `LANGUAGE` pragma (9.32.1) * `OPTIONS_GHC` pragma (9.32.2) * `WARNING` and `DEPRECATED` pragmas (9.32.4) * `LINE` pragma (9.32.7) * `COLUMN` pragma (9.32.8) * `SOURCE` pragma (9.32.14) * `glasgow_exts/generic_prog.rst`: * Generic classes (9.35) * Generic programming (9.36)