Portability | GHC |
---|---|

Stability | experimental |

Maintainer | bos@serpentine.com, rtomharper@googlemail.com, duncan@haskell.org |

Functions used frequently when reading textual data.

# Documentation

type Reader a = Text -> Either String (a, Text)Source

Read some text. If the read succeeds, return its value and the remaining text, otherwise an error message.

decimal :: Integral a => Reader aSource

Read a decimal integer. The input must begin with at least one decimal digit, and is consumed until a non-digit or end of string is reached.

This function does not handle leading sign characters. If you need
to handle signed input, use

.
`signed`

`decimal`

*Note*: For fixed-width integer types, this function does not
attempt to detect overflow, so a sufficiently long input may give
incorrect results. If you are worried about overflow, use
`Integer`

for your result type.

hexadecimal :: Integral a => Reader aSource

Read a hexadecimal integer, consisting of an optional leading
`"0x"`

followed by at least one decimal digit. Input is consumed
until a non-hex-digit or end of string is reached. This function
is case insensitive.

This function does not handle leading sign characters. If you need
to handle signed input, use

.
`signed`

`hexadecimal`

*Note*: For fixed-width integer types, this function does not
attempt to detect overflow, so a sufficiently long input may give
incorrect results. If you are worried about overflow, use
`Integer`

for your result type.

signed :: Num a => Reader a -> Reader aSource

Read an optional leading sign character (`'-'`

or `'+'`

) and
apply it to the result of applying the given reader.

rational :: RealFloat a => Reader aSource

Read a rational number.

This function accepts an optional leading sign character, followed
by at least one decimal digit. The syntax similar to that accepted
by the `read`

function, with the exception that a trailing `'.'`

or `'e'`

*not* followed by a number is not consumed.

Examples (with behaviour identical to `read`

):

rational "3" == Right (3.0, "") rational "3.1" == Right (3.1, "") rational "3e4" == Right (30000.0, "") rational "3.1e4" == Right (31000.0, "") rational ".3" == Left "input does not start with a digit" rational "e3" == Left "input does not start with a digit"

Examples of differences from `read`

:

rational "3.foo" == Right (3.0, ".foo") rational "3e" == Right (3.0, "e")

Read a rational number.

The syntax accepted by this function is the same as for `rational`

.

*Note*: This function is almost ten times faster than `rational`

,
but is slightly less accurate.

The `Double`

type supports about 16 decimal places of accuracy.
For 94.2% of numbers, this function and `rational`

give identical
results, but for the remaining 5.8%, this function loses precision
around the 15th decimal place. For 0.001% of numbers, this
function will lose precision at the 13th or 14th decimal place.