We define our own undefined which is marked as deprecated. This makes it useful to use during development, but lets you more easily get notifications if you accidentally ship partial code in production.

The classy prelude recommendation for when you need to really have a partial function in production is to use error with a very descriptive message so that, in case an exception is thrown, you get more information than Prelude.undefined.

Since 0.5.5

Provide a Semigroup for an arbitrary Monoid.

Only perform the action if the predicate returns True.

Since 0.9.2

Only perform the action if the predicate returns False.

Since 0.9.2

Generalized version of atomically.

Synonym for always.

Synonym for alwaysSucceeds.

Synonym for retry.

Synonym for orElse.

Synonym for check.

Class of monads which can perform primitive state-transformer actions

State token type

Convert a PrimBase to another monad with the same state token.

Convert a PrimBase with a RealWorld state token to IO

Convert a PrimBase to ST

Class of types supporting primitive array operations

The trace function outputs the trace message given as its first argument, before returning the second argument as its result.

For example, this returns the value of f x but first outputs the message.

trace ("calling f with x = " ++ show x) (f x)

The trace function should only be used for debugging, or for monitoring execution. The function is not referentially transparent: its type indicates that it is a pure function but it has the side effect of outputting the trace message.

Like trace, but uses show on the argument to convert it to a String.

This makes it convenient for printing the values of interesting variables or expressions inside a function. For example here we print the value of the variables x and z:

f x y =
    traceShow (x, z) $ result
  where
    z = ...
    ...

Since 0.5.9

Since 0.5.9

Since 0.5.9

Since 0.5.9

If the first argument evaluates to True, then the result is the second argument. Otherwise an AssertionFailed exception is raised, containing a String with the source file and line number of the call to assert.

Assertions can normally be turned on or off with a compiler flag (for GHC, assertions are normally on unless optimisation is turned on with -O or the -fignore-asserts option is given). When assertions are turned off, the first argument to assert is ignored, and the second argument is returned as the result.

Locale representing American usage.

knownTimeZones contains only the ten time-zones mentioned in RFC 822 sec. 5: "UT", "GMT", "EST", "EDT", "CST", "CDT", "MST", "MDT", "PST", "PDT". Note that the parsing functions will regardless parse single-letter military time-zones and +HHMM format.

Representable types of kind *. This class is derivable in GHC with the DeriveGeneric flag on.

Identity functor and monad. (a non-strict monad)

Since: 4.8.0.0

See examples in Control.Monad.Reader. Note, the partially applied function type (->) r is a simple reader monad. See the instance declaration below.

Retrieves the monad environment.

The reader monad transformer, which adds a read-only environment to the given monad.

The return function ignores the environment, while >>= passes the inherited environment to both subcomputations.

The parameterizable reader monad.

Computations are functions of a shared environment.

The return function ignores the environment, while >>= passes the inherited environment to both subcomputations.

Haskell defines operations to read and write characters from and to files, represented by values of type Handle. Each value of this type is a handle: a record used by the Haskell run-time system to manage I/O with file system objects. A handle has at least the following properties:

• whether it manages input or output or both;
• whether it is open, closed or semi-closed;
• whether the object is seekable;
• whether buffering is disabled, or enabled on a line or block basis;
• a buffer (whose length may be zero).

Most handles will also have a current I/O position indicating where the next input or output operation will occur. A handle is readable if it manages only input or both input and output; likewise, it is writable if it manages only output or both input and output. A handle is open when first allocated. Once it is closed it can no longer be used for either input or output, though an implementation cannot re-use its storage while references remain to it. Handles are in the Show and Eq classes. The string produced by showing a handle is system dependent; it should include enough information to identify the handle for debugging. A handle is equal according to == only to itself; no attempt is made to compare the internal state of different handles for equality.

A handle managing input from the Haskell program's standard input channel.

A handle managing output to the Haskell program's standard output channel.

A handle managing output to the Haskell program's standard error channel.

Repack from one type to another, dropping to a list in the middle.

repack = pack . unpack.

Since 0.9.2

Since 0.9.2

intercalate seq seqs inserts seq in between seqs and concatenates the result.

Since 0.9.3

same behavior as nub, but requires Hashable & Eq and is O(n log n)

https://github.com/nh2/haskell-ordnub

same behavior as nub, but requires Ord and is O(n log n)

https://github.com/nh2/haskell-ordnub

same behavior as nubBy, but requires Ord and is O(n log n)

https://github.com/nh2/haskell-ordnub

Sort elements using the user supplied function to project something out of each element. Inspired by http://hackage.haskell.org/packages/archive/base/latest/doc/html/GHC-Exts.html#v:sortWith.

repeat x is an infinite list, with x the value of every element.

An alias for difference.

An alias for intersection.

Conversion of values to readable Strings.

Derived instances of Show have the following properties, which are compatible with derived instances of Read:

• The result of show is a syntactically correct Haskell expression containing only constants, given the fixity declarations in force at the point where the type is declared. It contains only the constructor names defined in the data type, parentheses, and spaces. When labelled constructor fields are used, braces, commas, field names, and equal signs are also used.
• If the constructor is defined to be an infix operator, then showsPrec will produce infix applications of the constructor.
• the representation will be enclosed in parentheses if the precedence of the top-level constructor in x is less than d (associativity is ignored). Thus, if d is 0 then the result is never surrounded in parentheses; if d is 11 it is always surrounded in parentheses, unless it is an atomic expression.
• If the constructor is defined using record syntax, then show will produce the record-syntax form, with the fields given in the same order as the original declaration.

For example, given the declarations

infixr 5 :^:
data Tree a =  Leaf a  |  Tree a :^: Tree a

the derived instance of Show is equivalent to

instance (Show a) => Show (Tree a) where

       showsPrec d (Leaf m) = showParen (d > app_prec) $
            showString "Leaf " . showsPrec (app_prec+1) m
         where app_prec = 10

       showsPrec d (u :^: v) = showParen (d > up_prec) $
            showsPrec (up_prec+1) u .
            showString " :^: "      .
            showsPrec (up_prec+1) v
         where up_prec = 5

Note that right-associativity of :^: is ignored. For example,

• show (Leaf 1 :^: Leaf 2 :^: Leaf 3) produces the string "Leaf 1 :^: (Leaf 2 :^: Leaf 3)".

Convert a character to lower case.

Character-based case conversion is lossy in comparison to string-based toLower. For instance, İ will be converted to i, instead of i̇.

Convert a character to upper case.

Character-based case conversion is lossy in comparison to string-based toUpper. For instance, ß won't be converted to SS.

Data which can be read to and from files and handles.

Note that, for lazy sequences, these operations may perform lazy I/O.

Computation hClose hdl makes handle hdl closed. Before the computation finishes, if hdl is writable its buffer is flushed as for hFlush. Performing hClose on a handle that has already been closed has no effect; doing so is not an error. All other operations on a closed handle will fail. If hClose fails for any reason, any further operations (apart from hClose) on the handle will still fail as if hdl had been successfully closed.

Translates a FilePath to a Text This translation is not correct for a (unix) filename which can contain arbitrary (non-unicode) bytes: those bytes will be discarded.

This means you cannot translate the Text back to the original file name.

If you control or otherwise understand the filenames and believe them to be unicode valid consider using fpToTextEx or fpToTextWarn

Translates a FilePath to a Text

Warns if there are non-unicode sequences in the file name

Translates a FilePath to a Text

Throws an exception if there are non-unicode sequences in the file name

Use this to assert that you know a filename will translate properly into a Text. If you created the filename, this should be the case.

A difference list is a function that, given a list, returns the original contents of the difference list prepended to the given list.

This structure supports O(1) append and snoc operations on lists, making it very useful for append-heavy uses (esp. left-nested uses of ++), such as logging and pretty printing.

Here is an example using DList as the state type when printing a tree with the Writer monad:

import Control.Monad.Writer
import Data.DList

data Tree a = Leaf a | Branch (Tree a) (Tree a)

flatten_writer :: Tree x -> DList x
flatten_writer = snd . runWriter . flatten
    where
      flatten (Leaf x)     = tell (singleton x)
      flatten (Branch x y) = flatten x >> flatten y

Force type to a DList

Since 0.11.0

Synonym for apply

Since 0.11.0

A class for monads in which exceptions may be thrown.

Instances should obey the following law:

throwM e >> x = throwM e

In other words, throwing an exception short-circuits the rest of the monadic computation.

A class for monads which allow exceptions to be caught, in particular exceptions which were thrown by throwM.

Instances should obey the following law:

catch (throwM e) f = f e

Note that the ability to catch an exception does not guarantee that we can deal with all possible exit points from a computation. Some monads, such as continuation-based stacks, allow for more than just a success/failure strategy, and therefore catch cannot be used by those monads to properly implement a function such as finally. For more information, see MonadMask.

A class for monads which provide for the ability to account for all possible exit points from a computation, and to mask asynchronous exceptions. Continuation-based monads, and stacks such as ErrorT e IO which provide for multiple failure modes, are invalid instances of this class.

Note that this package does provide a MonadMask instance for CatchT. This instance is only valid if the base monad provides no ability to provide multiple exit. For example, IO or Either would be invalid base monads, but Reader or State would be acceptable.

Instances should ensure that, in the following code:

f `finally` g

The action g is called regardless of what occurs within f, including async exceptions.