read -package:tar package:base

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

Read up to the specified number of bytes starting from a specified offset, returning the number of bytes actually read. This function should only block if there is no data available. If there is not enough data available, then the function should just return the available data. A return value of zero indicates that the end of the data stream (e.g. end of file) has been reached.

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

Converting strings to values. The Text.Read library is the canonical library to import for Read-class facilities. For GHC only, it offers an extended and much improved Read class, which constitutes a proposed alternative to the Haskell 2010 Read. In particular, writing parsers is easier, and the parsers are much more efficient.

The Read class and instances for basic data types.

The readFile function reads a file and returns the contents of the file as a string. The file is read lazily, on demand, as with getContents.

The readIO function is similar to read except that it signals parse failure to the IO monad instead of terminating the program.

The method readList is provided to allow the programmer to give a specialised way of parsing lists of values. For example, this is used by the predefined Read instance of the Char type, where values of type String should be are expected to use double quotes, rather than square brackets.

The readLn function combines getLine and readIO.

readParen True p parses what p parses, but surrounded with parentheses. readParen False p parses what p parses, but optionally surrounded with parentheses.

equivalent to readsPrec with a precedence of 0.

attempts to parse a value from the front of the string, returning a list of (parsed value, remaining string) pairs. If there is no successful parse, the returned list is empty. Derived instances of Read and Show satisfy the following:

• (x,"") is an element of (readsPrec d (showsPrec d x "")).

That is, readsPrec parses the string produced by showsPrec, and delivers the value that showsPrec started with.

Read the next value from the Chan. Blocks when the channel is empty. Since the read end of a channel is an MVar, this operation inherits fairness guarantees of MVars (e.g. threads blocked in this operation are woken up in FIFO order). Throws BlockedIndefinitelyOnMVar when the channel is empty and no other thread holds a reference to the channel.

Atomically read the contents of an MVar. If the MVar is currently empty, readMVar will wait until it is full. readMVar is guaranteed to receive the next putMVar. readMVar is multiple-wakeup, so when multiple readers are blocked on an MVar, all of them are woken up at the same time. Compatibility note: Prior to base 4.7, readMVar was a combination of takeMVar and putMVar. This mean that in the presence of other threads attempting to putMVar, readMVar could block. Furthermore, readMVar would not receive the next putMVar if there was already a pending thread blocked on takeMVar. The old behavior can be recovered by implementing 'readMVar as follows:

readMVar :: MVar a -> IO a
readMVar m =
mask_ $ do
a <- takeMVar m
putMVar m a
return a

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")]

Lookup a constructor via a string

readBinaryWith rp1 rp2 n c' matches the name of a binary data constructor and then parses its arguments using rp1 and rp2 respectively.

readData p is a parser for datatypes where each alternative begins with a data constructor. It parses the constructor and passes it to p. Parsers for various constructors can be constructed with readUnaryWith and readBinaryWith, and combined with (<|>) from the Alternative class.

Lift the standard readPrec and readListPrec functions through the type constructor.

Lift the standard readPrec function through the type constructor.

readUnaryWith rp n c' matches the name of a unary data constructor and then parses its argument using rp.

Deprecated: Use readsBinaryWith to define liftReadsPrec

readsBinaryWith rp1 rp2 n c n' matches the name of a binary data constructor and then parses its arguments using rp1 and rp2 respectively.

readsData p d is a parser for datatypes where each alternative begins with a data constructor. It parses the constructor and passes it to p. Parsers for various constructors can be constructed with readsUnary, readsUnary1 and readsBinary1, and combined with mappend from the Monoid class.

Lift the standard readsPrec and readList functions through the type constructor.