Read package:Cabal-syntax

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

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).

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.

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

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.

Parse cabal style ByteString into list of Fields, i.e. the cabal AST. readFields assumes that input ByteString is valid UTF8, specifically it doesn't validate that file is valid UTF8. Therefore bytestrings inside returned Field will be invalid as UTF8 if the input were.

>>> readFields "foo: \223"
Right [Field (Name (Position 1 1) "foo") [FieldLine (Position 1 6) "\223"]]

readFields won't (necessarily) fail on invalid UTF8 data, but the reported positions may be off. You may get weird errors on non-UTF8 input, for example readFields will fail on latin1 encoded non-breaking space:

>>> isLeft (readFields "\xa0 foo: bar")
True

That is rejected because parser thinks \xa0 is a section name, and section arguments may not contain colon. If there are just latin1 non-breaking spaces, they become part of the name:

>>> readFields "\xa0\&foo: bar"
Right [Field (Name (Position 1 1) "\160foo") [FieldLine (Position 1 7) "bar"]]

The UTF8 non-breaking space is accepted as an indentation character (but warned about by readFields').

>>> readFields' "\xc2\xa0 foo: bar"
Right ([Field (Name (Position 1 3) "foo") [FieldLine (Position 1 8) "bar"]],[LexWarning LexWarningNBSP (Position 1 1)])

Like readFields but also return lexer warnings.

Reads a UTF8 encoded text file as a Unicode String Reads lazily using ordinary readFile.

TermReadKey, TermReadKey License, SPDX License List 3.23, SPDX License List 3.25