base- Basic libraries

Copyright(c) The University of Glasgow 2002
Licensesee libraries/base/LICENSE
Portabilitynon-portable (GHC Extensions)
Safe HaskellUnsafe




GHC Extensions: this is the Approved Way to get at GHC-specific extensions.

Note: no other base module should import this module.


Representations of some basic types

data Int :: *

A fixed-precision integer type with at least the range [-2^29 .. 2^29-1]. The exact range for a given implementation can be determined by using minBound and maxBound from the Bounded class.


I# Int# 

data Word :: *

A Word is an unsigned integral type, with the same size as Int.


W# Word# 

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.


F# Float# 

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.


D# Double# 

data Char :: *

The character type Char is an enumeration whose values represent Unicode (or equivalently ISO/IEC 10646) characters (see 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).


C# Char# 

data Ptr a Source

A value of type Ptr a represents a pointer to an object, or an array of objects, which may be marshalled to or from Haskell values of type a.

The type a will often be an instance of class Storable which provides the marshalling operations. However this is not essential, and you can provide your own operations to access the pointer. For example you might write small foreign functions to get or set the fields of a C struct.


Ptr Addr# 


Typeable1 Ptr 
Eq (Ptr a) 
(Data a, Typeable * a) => Data (Ptr a) 
Ord (Ptr a) 
Show (Ptr a) 
Storable (Ptr a) 
Typeable (* -> *) Ptr 

data FunPtr a Source

A value of type FunPtr a is a pointer to a function callable from foreign code. The type a will normally be a foreign type, a function type with zero or more arguments where

A value of type FunPtr a may be a pointer to a foreign function, either returned by another foreign function or imported with a a static address import like

foreign import ccall "stdlib.h &free"
  p_free :: FunPtr (Ptr a -> IO ())

or a pointer to a Haskell function created using a wrapper stub declared to produce a FunPtr of the correct type. For example:

type Compare = Int -> Int -> Bool
foreign import ccall "wrapper"
  mkCompare :: Compare -> IO (FunPtr Compare)

Calls to wrapper stubs like mkCompare allocate storage, which should be released with freeHaskellFunPtr when no longer required.

To convert FunPtr values to corresponding Haskell functions, one can define a dynamic stub for the specific foreign type, e.g.

type IntFunction = CInt -> IO ()
foreign import ccall "dynamic" 
  mkFun :: FunPtr IntFunction -> IntFunction


FunPtr Addr# 


The maximum tuple size

Primitive operations

module GHC.Prim

shiftL# :: Word# -> Int# -> Word# Source

Shift the argument left by the specified number of bits (which must be non-negative).

shiftRL# :: Word# -> Int# -> Word# Source

Shift the argument right by the specified number of bits (which must be non-negative).

iShiftL# :: Int# -> Int# -> Int# Source

Shift the argument left by the specified number of bits (which must be non-negative).

iShiftRA# :: Int# -> Int# -> Int# Source

Shift the argument right (signed) by the specified number of bits (which must be non-negative).

iShiftRL# :: Int# -> Int# -> Int# Source

Shift the argument right (unsigned) by the specified number of bits (which must be non-negative).

isTrue# :: Int# -> Bool

Alias for tagToEnum#. Returns True of its parameter is 1# and False if it is 0#.


build :: forall a. (forall b. (a -> b -> b) -> b -> b) -> [a] Source

A list producer that can be fused with foldr. This function is merely

   build g = g (:) []

but GHC's simplifier will transform an expression of the form foldr k z (build g), which may arise after inlining, to g k z, which avoids producing an intermediate list.

augment :: forall a. (forall b. (a -> b -> b) -> b -> b) -> [a] -> [a] Source

A list producer that can be fused with foldr. This function is merely

   augment g xs = g (:) xs

but GHC's simplifier will transform an expression of the form foldr k z (augment g xs), which may arise after inlining, to g k (foldr k z xs), which avoids producing an intermediate list.

Overloaded string literals

class IsString a where Source

Class for string-like datastructures; used by the overloaded string extension (-XOverloadedStrings in GHC).


fromString :: String -> a Source



breakpoint :: a -> a Source

Ids with special behaviour

lazy :: a -> a

The lazy function restrains strictness analysis a little. The call '(lazy e)' means the same as e, but lazy has a magical property so far as strictness analysis is concerned: it is lazy in its first argument, even though its semantics is strict. After strictness analysis has run, calls to lazy are inlined to be the identity function.

This behaviour is occasionally useful when controlling evaluation order. Notably, lazy is used in the library definition of par:

par :: a -> b -> b
par x y = case (par# x) of _ -> lazy y

If lazy were not lazy, par would look strict in y which would defeat the whole purpose of par.

Like seq, the argument of lazy can have an unboxed type.

inline :: a -> a

The call '(inline f)' arranges that f is inlined, regardless of its size. More precisely, the call '(inline f)' rewrites to the right-hand side of 'f'\'s definition. This allows the programmer to control inlining from a particular call site rather than the definition site of the function (c.f. INLINE pragmas).

This inlining occurs regardless of the argument to the call or the size of 'f'\'s definition; it is unconditional. The main caveat is that 'f'\'s definition must be visible to the compiler; it is therefore recommended to mark the function with an INLINABLE pragma at its definition so that GHC guarantees to record its unfolding regardless of size.

If no inlining takes place, the inline function expands to the identity function in Phase zero, so its use imposes no overhead.

Safe coercions

These are available from the Trustworthy module Data.Coerce as well


coerce :: Coercible k a b => a -> b

The function coerce allows you to safely convert between values of types that have the same representation with no run-time overhead. In the simplest case you can use it instead of a newtype constructor, to go from the newtype's concrete type to the abstract type. But it also works in more complicated settings, e.g. converting a list of newtypes to a list of concrete types.

class 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.


Transform comprehensions

newtype Down a Source

The Down type allows you to reverse sort order conveniently. A value of type Down a contains a value of type a (represented as Down a). If a has an Ord instance associated with it then comparing two values thus wrapped will give you the opposite of their normal sort order. This is particularly useful when sorting in generalised list comprehensions, as in: then sortWith by Down x

Provides Show and Read instances (since:



Down a 


Eq a => Eq (Down a) 
Ord a => Ord (Down a) 
Read a => Read (Down a) 
Show a => Show (Down a) 

groupWith :: Ord b => (a -> b) -> [a] -> [[a]] Source

The groupWith function uses the user supplied function which projects an element out of every list element in order to first sort the input list and then to form groups by equality on these projected elements

sortWith :: Ord b => (a -> b) -> [a] -> [a] Source

The sortWith function sorts a list of elements using the user supplied function to project something out of each element

the :: Eq a => [a] -> a Source

the ensures that all the elements of the list are identical and then returns that unique element

Event logging

traceEvent :: String -> IO () Source

Deprecated: Use traceEvent or traceEventIO

SpecConstr annotations

The call stack

currentCallStack :: IO [String] Source

returns a '[String]' representing the current call stack. This can be useful for debugging.

The implementation uses the call-stack simulation maintined by the profiler, so it only works if the program was compiled with -prof and contains suitable SCC annotations (e.g. by using -fprof-auto). Otherwise, the list returned is likely to be empty or uninformative.


The Constraint kind

data Constraint :: BOX


Overloaded lists

class IsList l where Source

The IsList class and its methods are intended to be used in conjunction with the OverloadedLists extension.


Minimal complete definition

fromList, toList

Associated Types

type Item l Source

The Item type function returns the type of items of the structure l.


fromList :: [Item l] -> l Source

The fromList function constructs the structure l from the given list of Item l

fromListN :: Int -> [Item l] -> l Source

The fromListN function takes the input list's length as a hint. Its behaviour should be equivalent to fromList. The hint can be used to construct the structure l more efficiently compared to fromList. If the given hint does not equal to the input list's length the behaviour of fromListN is not specified.

toList :: l -> [Item l] Source

The toList function extracts a list of Item l from the structure l. It should satisfy fromList . toList = id.


IsList [a]