# Documentation

`Repr a`

is a value of type `a`

paired with a way to render that value to a
string which will contain a representation of the value.

Note that `Repr a`

is overloaded for all the numeric classes provided that
`a`

has instances for the respected classes. This allows you to write a
numeric expression of type `Repr a`

. For example:

*Repr> let rd = 1.5 + 2 + (3 + (-4) * (5 - pi / sqrt 6)) :: Repr Double

You can extract the value of `rd`

:

*Repr> value rd 17.281195923884734

And you can than render `rd`

to its textual representation:

*Repr> render rd "fromRational (3 % 2) + fromInteger 2 + (fromInteger 3 + negate (fromInteger 4) * (fromInteger 5 - pi / sqrt (fromInteger 6)))"

Enum a => Enum (Repr a) | |

Eq a => Eq (Repr a) | |

Floating a => Floating (Repr a) | |

Fractional a => Fractional (Repr a) | |

Integral a => Integral (Repr a) | |

Num a => Num (Repr a) | |

Ord a => Ord (Repr a) | |

Real a => Real (Repr a) | |

RealFloat a => RealFloat (Repr a) | |

RealFrac a => RealFrac (Repr a) | |

Show (Repr a) | |

IsString a => IsString (Repr a) |

type Renderer = Precedence -> Fixity -> DStringSource

To render you need to supply the precedence and fixity of the enclosing context.

For more documentation about precedence and fixity see:

http://haskell.org/onlinereport/decls.html#sect4.4.2

The reason the renderer returns a `DString`

instead of for example a `String`

is that the rendering of numeric expression involves lots of left-factored
appends i.e.: `((a ++ b) ++ c) ++ d`

. A `DString`

has a O(1) append operation
while a `String`

just has a O(n) append. So choosing a `DString`

is more
efficient.

type Precedence = IntSource

The precedence of operators and function application.

- Operators usually have a precedence in the range of 0 to 9.
- Function application always has precedence 10.

Fixity of operators.

(<?>) :: Repr a -> DString -> Repr aSource

`x <?> s`

annotates the rendering with the given string.

The output wil look like: `"({- s -} ...)"`

where `...`

is the rendering of
`x`

.

This combinator is handy when you want to render the ouput of a function and
you want to see how the parameters of the function contribute to the
result. For example, suppose you defined the following function `f`

:

f p0 p1 p2 = p0 ^ 2 + sqrt p1 * ([p2..] !! 10)

You can then apply `f`

to some parameters annotated with some descriptive
strings (the name of the parameter is usally a good idea):

f (1 <?> "p0") (2 <?> "p1") (3 <?> "p2")

The rendering will then look like:

"({- p0 -} fromInteger 1) * ({- p0 -} fromInteger 1) + sqrt ({- p1 -} (fromInteger 2)) * enumFrom ({- p2 -} (fromInteger 3)) !! 10"