read -package:base-prelude -package:HsOpenSSL package:rio

The read function reads input from a string, which must be completely consumed by the input process. read fails with an error if the parse is unsuccessful, and it is therefore discouraged from being used in real applications. Use readMaybe or readEither for safe alternatives.

>>> read "123" :: Int
123

>>> read "hello" :: Int
*** Exception: Prelude.read: no parse

Parsing of Strings, producing values. Derived instances of Read make the following assumptions, which derived instances of Show obey:

• If the constructor is defined to be an infix operator, then the derived Read instance will parse only infix applications of the constructor (not the prefix form).
• Associativity is not used to reduce the occurrence of parentheses, although precedence may be.
• If the constructor is defined using record syntax, the derived Read will parse only the record-syntax form, and furthermore, the fields must be given in the same order as the original declaration.
• The derived Read instance allows arbitrary Haskell whitespace between tokens of the input string. Extra parentheses are also allowed.

For example, given the declarations

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

the derived instance of Read in Haskell 2010 is equivalent to

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

readsPrec d r =  readParen (d > app_prec)
(\r -> [(Leaf m,t) |
("Leaf",s) <- lex r,
(m,t) <- readsPrec (app_prec+1) s]) r

++ readParen (d > up_prec)
(\r -> [(u:^:v,w) |
(u,s) <- readsPrec (up_prec+1) r,
(":^:",t) <- lex s,
(v,w) <- readsPrec (up_prec+1) t]) r

where app_prec = 10
up_prec = 5

Note that right-associativity of :^: is unused. The derived instance in GHC is equivalent to

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

readPrec = parens $ (prec app_prec $ do
Ident "Leaf" <- lexP
m <- step readPrec
return (Leaf m))

+++ (prec up_prec $ do
u <- step readPrec
Symbol ":^:" <- lexP
v <- step readPrec
return (u :^: v))

where app_prec = 10
up_prec = 5

readListPrec = readListPrecDefault

Why do both readsPrec and readPrec exist, and why does GHC opt to implement readPrec in derived Read instances instead of readsPrec? The reason is that readsPrec is based on the ReadS type, and although ReadS is mentioned in the Haskell 2010 Report, it is not a very efficient parser data structure. readPrec, on the other hand, is based on a much more efficient ReadPrec datatype (a.k.a "new-style parsers"), but its definition relies on the use of the RankNTypes language extension. Therefore, readPrec (and its cousin, readListPrec) are marked as GHC-only. Nevertheless, it is recommended to use readPrec instead of readsPrec whenever possible for the efficiency improvements it brings. As mentioned above, derived Read instances in GHC will implement readPrec instead of readsPrec. The default implementations of readsPrec (and its cousin, readList) will simply use readPrec under the hood. If you are writing a Read instance by hand, it is recommended to write it like so:

instance Read T where
readPrec     = ...
readListPrec = readListPrecDefault

Same as readFile, but generalized to MonadIO

Read a file in UTF8 encoding, throwing an exception on invalid character encoding. This function will use OS-specific line ending handling.

Read from a SomeRef

Read the value in a URef

Lifted readFile

Lifted readFile

Read a string representation of a character, using Haskell source-language escape conventions, and convert it to the character that it encodes. For example:

readLitChar "\\nHello"  =  [('\n', "Hello")]

Parse a string using the Read instance. Succeeds if there is exactly one valid result.

>>> readMaybe "123" :: Maybe Int
Just 123

>>> readMaybe "hello" :: Maybe Int
Nothing

Run a process, capture its standard output and error as a ByteString, wait for it to complete, and then return its exit code, output, and error. Note that any previously used setStdout or setStderr will be overridden.

Same as readProcess, but only read the stderr of the process. Original settings for stdout remain.

Same as readProcessStderr, but instead of returning the ExitCode, checks it with checkExitCode. Exceptions thrown by this function will include stderr.

Same as readProcess, but only read the stdout of the process. Original settings for stderr remain.

Same as readProcessStdout, but instead of returning the ExitCode, checks it with checkExitCode. Exceptions thrown by this function will include stdout.

Same as readProcess, but instead of returning the ExitCode, checks it with checkExitCode. Exceptions thrown by this function will include stdout and stderr.

Parse a time value given a format string. See parseTimeM for details.

Parse a time value given a format string. See parseTimeM for details.

Generic definition of readPrec

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.

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.

A ThreadId is an abstract type representing a handle to a thread. ThreadId is an instance of Eq, Ord and Show, where the Ord instance implements an arbitrary total ordering over ThreadIds. The Show instance lets you convert an arbitrary-valued ThreadId to string form; showing a ThreadId value is occasionally useful when debugging or diagnosing the behaviour of a concurrent program. Note: in GHC, if you have a ThreadId, you essentially have a pointer to the thread itself. This means the thread itself can't be garbage collected until you drop the ThreadId. This misfeature will hopefully be corrected at a later date.