# Documentation

`Repr α`

is a value of type `α`

paired with a way to render that value to
its textual representation.

`Repr`

s follow the property that given a `Repr`

`r`

if you evaluate the textual
representation of `r`

you should get the value or `r`

.

Note that `Repr α`

has an instance for most classes in `base`

provided that `α`

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

. For example:

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

You can extract the value of `r`

:

*Repr> extract r 17.281195923884734

And you can render `r`

to its textual representation using `show`

:

*Repr> show r "fromRational (3 % 2) + 2 + (3 + negate 4 * (5 - pi / sqrt 6))"

Bounded α => Bounded (Repr α) | |

Enum α => Enum (Repr α) | |

Eq α => Eq (Repr α) | |

Floating α => Floating (Repr α) | |

Fractional α => Fractional (Repr α) | |

Integral α => Integral (Repr α) | |

Num α => Num (Repr α) | |

Ord α => Ord (Repr α) | |

Read α => Read (Repr α) | |

Real α => Real (Repr α) | |

RealFloat α => RealFloat (Repr α) | |

RealFrac α => RealFrac (Repr α) | |

Show (Repr α) | |

Ix α => Ix (Repr α) | |

Typeable α => Typeable (Repr α) | |

IsString α => IsString (Repr α) | |

HasResolution α => HasResolution (Repr α) | |

Monoid α => Monoid (Repr α) | |

(Show α, Storable α) => Storable (Repr α) | |

Bits α => Bits (Repr α) | |

Exception α => Exception (Repr α) | |

ToString α => ToString (Repr α) | |

(Random α, Show α) => Random (Repr α) |

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`

has to do with efficiency. The rendering of expressions involves lots of
left-factored appends i.e.: `((a ++ b) ++ c) ++ d`

. A `DString`

, which is
equivalent to a `ShowS`

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

has a O(n)
append.

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 α -> DString -> Repr αSource

`x <?> s`

annotates the rendering with the given string.

The rendering 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"