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| -rw-r--r-- | docs/users_guide/exts/rebindable_syntax.rst | 45 |
1 files changed, 42 insertions, 3 deletions
diff --git a/docs/users_guide/exts/rebindable_syntax.rst b/docs/users_guide/exts/rebindable_syntax.rst index 3b41d5097b..4955e17c11 100644 --- a/docs/users_guide/exts/rebindable_syntax.rst +++ b/docs/users_guide/exts/rebindable_syntax.rst @@ -14,9 +14,7 @@ Rebindable syntax and the implicit Prelude import GHC normally imports ``Prelude.hi`` files for you. If you'd rather it didn't, then give it a ``-XNoImplicitPrelude`` -option. The idea is that you can then import a Prelude of your own. (But -don't call it ``Prelude``; the Haskell module namespace is flat, and you -must not conflict with any Prelude module.) +option. The idea is that you can then import a Prelude of your own. .. extension:: RebindableSyntax :shortdesc: Employ rebindable syntax. @@ -91,6 +89,47 @@ Be warned: this is an experimental facility, with fewer checks than usual. Use ``-dcore-lint`` to typecheck the desugared program. If Core Lint is happy you should be all right. +Custom Prelude modules named ``Prelude`` +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +If you call your custom Prelude module ``Prelude`` and place it in a file called +``Prelude.hs``, then your custom Prelude will be implicitly imported instead of +the default Prelude. + +Here is an example that compiles: :: + + $ cat Prelude.hs + module Prelude where + + a = () + + $ cat B.hs + module B where + + foo = a + + $ ghc Prelude.hs B.hs + [1 of 2] Compiling Prelude ( Prelude.hs, Prelude.o ) + [2 of 2] Compiling B ( B.hs, B.o ) + +The new ``Prelude`` is implicitly imported in ``B.hs``. + +Here is an example that does not compile: :: + + $ cat Prelude.hs + module Prelude where + + foo = True + + $ ghc Prelude.hs + [1 of 1] Compiling Prelude ( Prelude.hs, Prelude.o ) + + Prelude.hs:3:7: error: Data constructor not in scope: True + +The original ``Prelude`` module is shadowed by the custom Prelude in this case. +To include the original Prelude in your custom Prelude, you can explicitly +import it with the ``-XPackageImports`` option and ``import "base" Prelude``. + Things unaffected by :extension:`RebindableSyntax` ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |