Diagrams.Util

Some miscellaneous utilities provided by the diagrams-lib package.

Utilities for users

with :: Default d => d

Several functions exported by the diagrams library take a number of arguments giving the user control to "tweak" various aspects of their behavior. Rather than give such functions a long list of arguments, and to make it possible for the user to selectively override only certain arguments and use default values for others, such sets of arguments are collected into a record with named fields (see PolygonOpts in Diagrams.TwoD.Shapes for an example). Such record types are made instances of the Default class, which provides a single record structure (def) collecting the "default" arguments to the function. with is a synonym for def, which provides nice-looking syntax for simulating optional, named arguments in Haskell. For example,

 polygon with {sides = 7, edgeSkip = 2}

calls the polygon function with a single argument (note that record update binds more tightly than function application!), namely, with (the record of default arguments) where the sides and edgeSkip fields have been updated.

(<>) :: Monoid m => m -> m -> m

A convenient infix operator for mappend (monoidal combination). Many things in the diagrams library can be combined using (<>), with the meaning dependent on the types of things being combined. For example:

• The combination of two transformations t1 <> t2 is a transformation which performs first t2, then t1.
• Combining two diagrams d1 <> d2 results in a superimposed diagram with d1 on top of d2 (with their local origins aligned).
• Combining two paths works in the same way as combining diagrams.
• Combining two trails results in a longer trail composed of the first trail followed by the second.
• Combining two styles, s1 <> s2, results in a style with combined attributes from both, biased to s2 when s1 and s2 contain attributes of the same type.
• Combining two AlphaColour Doubles results in a composited color (the color that results when objects of the two colors are superimposed).

In addition, mempty always represents a suitably "trivial" object which is the identity for (<>) (that is, mempty <> x == x <> mempty == x). mempty can stand for the identity transformation; the empty diagram, path, trail, or style; the completely transparent color; and so on.

(#) :: a -> (a -> b) -> b

Postfix function application, for conveniently applying attributes. Unlike ($), (#) has a high precedence (8), so d # foo # bar can be combined with other things using operators like (|||) or (<>) without needing parentheses.

Internal utilities

data Proxy a

A value of Proxy a carries no information; it's used only to fix the type a.

Constructors

Proxy