| Copyright | (c) The University of Glasgow 2009 |
|---|---|
| License | see libraries/ghc-prim/LICENSE |
| Maintainer | cvs-ghc@haskell.org |
| Stability | internal |
| Portability | non-portable (GHC Extensions) |
| Safe Haskell | None |
| Language | Haskell2010 |
GHC.Types
Description
GHC type definitions. Use GHC.Exts from the base package instead of importing this module directly.
Documentation
data Char
The character type Char is an enumeration whose values represent
Unicode (or equivalently ISO/IEC 10646) characters (see
http://www.unicode.org/ for details). This set extends the ISO 8859-1
(Latin-1) character set (the first 256 characters), which is itself an extension
of the ASCII character set (the first 128 characters). A character literal in
Haskell has type Char.
To convert a Char to or from the corresponding Int value defined
by Unicode, use toEnum and fromEnum from the
Enum class respectively (or equivalently ord and chr).
data Int
data Float
Single-precision floating point numbers. It is desirable that this type be at least equal in range and precision to the IEEE single-precision type.
data Double
Double-precision floating point numbers. It is desirable that this type be at least equal in range and precision to the IEEE double-precision type.
newtype IO a
A value of type is a computation which, when performed,
does some I/O before returning a value of type IO aa.
There is really only one way to "perform" an I/O action: bind it to
Main.main in your program. When your program is run, the I/O will
be performed. It isn't possible to perform I/O from an arbitrary
function, unless that function is itself in the IO monad and called
at some point, directly or indirectly, from Main.main.
IO is a monad, so IO actions can be combined using either the do-notation
or the >> and >>= operations from the Monad class.
Alias for tagToEnum#. Returns True if its parameter is 1# and False
if it is 0#.
data SPEC
SPEC is used by GHC in the SpecConstr pass in order to inform
the compiler when to be particularly aggressive. In particular, it
tells GHC to specialize regardless of size or the number of
specializations. However, not all loops fall into this category.
Libraries can specify this by using SPEC data type to inform which
loops should be aggressively specialized.
data Coercible a b
This two-parameter class has instances for types a and b if
the compiler can infer that they have the same representation. This class
does not have regular instances; instead they are created on-the-fly during
type-checking. Trying to manually declare an instance of Coercible
is an error.
Nevertheless one can pretend that the following three kinds of instances exist. First, as a trivial base-case:
instance a a
Furthermore, for every type constructor there is
an instance that allows to coerce under the type constructor. For
example, let D be a prototypical type constructor (data or
newtype) with three type arguments, which have roles nominal,
representational resp. phantom. Then there is an instance of
the form
instance Coercible b b' => Coercible (D a b c) (D a b' c')
Note that the nominal type arguments are equal, the
representational type arguments can differ, but need to have a
Coercible instance themself, and the phantom type arguments can be
changed arbitrarily.
The third kind of instance exists for every newtype NT = MkNT T and
comes in two variants, namely
instance Coercible a T => Coercible a NT
instance Coercible T b => Coercible NT b
This instance is only usable if the constructor MkNT is in scope.
If, as a library author of a type constructor like Set a, you
want to prevent a user of your module to write
coerce :: Set T -> Set NT,
you need to set the role of Set's type parameter to nominal,
by writing
type role Set nominal
For more details about this feature, please refer to Safe Coercions by Joachim Breitner, Richard A. Eisenberg, Simon Peyton Jones and Stephanie Weirich.
Since: 4.7.0.0