6.2.10. Rebindable syntax and the implicit Prelude import

NoImplicitPrelude
Implied by:RebindableSyntax
Since:6.8.1

Don’t import Prelude by default.

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.

RebindableSyntax
Implies:NoImplicitPrelude
Since:7.0.1

Enable rebinding of a variety of usually-built-in operations.

Suppose you are importing a Prelude of your own in order to define your own numeric class hierarchy. It completely defeats that purpose if the literal “1” means “Prelude.fromInteger 1”, which is what the Haskell Report specifies. So the RebindableSyntax extension causes the following pieces of built-in syntax to refer to whatever is in scope, not the Prelude versions:

  • An integer literal 368 means “fromInteger (368::Integer)”, rather than “Prelude.fromInteger (368::Integer)”.
  • Fractional literals are handled in just the same way, except that the translation is fromRational (3.68::Rational).
  • String literals are also handled the same way, except that the translation is fromString ("368"::String).
  • The equality test in an overloaded numeric pattern uses whatever (==) is in scope.
  • The subtraction operation, and the greater-than-or-equal test, in n+k patterns use whatever (-) and (>=) are in scope.
  • Negation (e.g. “- (f x)”) means “negate (f x)”, both in numeric patterns, and expressions.
  • Conditionals (e.g. “if e1 then e2 else e3”) means “ifThenElse e1 e2 e3”. However case expressions are unaffected.
  • “Do” notation is translated using whatever functions (>>=), (>>), and fail, are in scope (not the Prelude versions). List comprehensions, mdo (The recursive do-notation), and parallel array comprehensions, are unaffected.
  • Arrow notation (see Arrow notation) uses whatever arr, (>>>), first, app, (|||) and loop functions are in scope. But unlike the other constructs, the types of these functions must match the Prelude types very closely. Details are in flux; if you want to use this, ask!
  • List notation, such as [x,y] or [m..n] can also be treated via rebindable syntax if you use -XOverloadedLists; see Overloaded lists.
  • An overloaded label “#foo” means “fromLabel @"foo"”, rather than “GHC.OverloadedLabels.fromLabel @"foo"” (see Overloaded labels).

RebindableSyntax implies NoImplicitPrelude.

In all cases (apart from arrow notation), the static semantics should be that of the desugared form, even if that is a little unexpected. For example, the static semantics of the literal 368 is exactly that of fromInteger (368::Integer); it’s fine for fromInteger to have any of the types:

fromInteger :: Integer -> Integer
fromInteger :: forall a. Foo a => Integer -> a
fromInteger :: Num a => a -> Integer
fromInteger :: Integer -> Bool -> Bool

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.

6.2.10.1. 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.

6.2.10.2. Things unaffected by RebindableSyntax

RebindableSyntax does not apply to any code generated from a deriving clause or declaration. To see why, consider the following code:

{-# LANGUAGE RebindableSyntax, OverloadedStrings #-}
newtype Text = Text String

fromString :: String -> Text
fromString = Text

data Foo = Foo deriving Show

This will generate code to the effect of:

instance Show Foo where
  showsPrec _ Foo = showString "Foo"

But because RebindableSyntax and OverloadedStrings are enabled, the "Foo" string literal would now be of type Text, not String, which showString doesn’t accept! This causes the generated Show instance to fail to typecheck. It’s hard to imagine any scenario where it would be desirable have RebindableSyntax behavior within derived code, so GHC simply ignores RebindableSyntax entirely when checking derived code.

6.2.11. Postfix operators

PostfixOperators
Since:7.10.1
Status:Included in GHC2021

Allow the use of post-fix operators

The PostfixOperators extension enables a small extension to the syntax of left operator sections, which allows you to define postfix operators. The extension is this: for any expression e and operator (!), the left section

(e !)

is equivalent (from the point of view of both type checking and execution) to the expression

((!) e)

The strict Haskell 98 interpretation is that the section is equivalent to

(\y -> (!) e y)

That is, the operator must be a function of two arguments. GHC allows it to take only one argument, and that in turn allows you to write the function postfix.

The extension does not extend to the left-hand side of function definitions; you must define such a function in prefix form.