Read package:base-prelude

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

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

Lifting of the Read class to unary type constructors. Both liftReadsPrec and liftReadPrec exist to match the interface provided in the Read type class, but it is recommended to implement Read1 instances using liftReadPrec as opposed to liftReadsPrec, since the former is more efficient than the latter. For example:

instance Read1 T where
liftReadPrec     = ...
liftReadListPrec = liftReadListPrecDefault

For more information, refer to the documentation for the Read class.

Lifting of the Read class to binary type constructors. Both liftReadsPrec2 and liftReadPrec2 exist to match the interface provided in the Read type class, but it is recommended to implement Read2 instances using liftReadPrec2 as opposed to liftReadsPrec2, since the former is more efficient than the latter. For example:

instance Read2 T where
liftReadPrec2     = ...
liftReadListPrec2 = liftReadListPrec2Default

For more information, refer to the documentation for the Read class.

A parser for a type a, represented as a function that takes a String and returns a list of possible parses as (a,String) pairs. Note that this kind of backtracking parser is very inefficient; reading a large structure may be quite slow (cf ReadP).

Read an unsigned number in binary notation.

>>> readBin "10011"
[(19,"")]

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

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.

Lookup a constructor via a string

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.

Read an unsigned number in decimal notation.

>>> readDec "0644"
[(644,"")]

Parse a string using the Read instance. Succeeds if there is exactly one valid result. A Left value indicates a parse error.

>>> readEither "123" :: Either String Int
Right 123

>>> readEither "hello" :: Either String Int
Left "Prelude.read: no parse"

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 readFile' function reads a file and returns the contents of the file as a string. The file is fully read before being returned, as with getContents'.

Reads an unsigned RealFrac value, expressed in decimal scientific notation.

Read an unsigned number in hexadecimal notation. Both upper or lower case letters are allowed.

>>> readHex "deadbeef"
[(3735928559,"")]

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

Read the value of an IORef

Reads an unsigned Integral value in an arbitrary base.

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.

Proposed replacement for readList using new-style parsers (GHC only). The default definition uses readList. Instances that define readPrec should also define readListPrec as readListPrecDefault.