7  Pan# For Haskell Programmers

Pan# does not allow new user-defined data types, only type synonyms. Only tuples are available for data structuring. Operations cannot be overloaded.

7.1  Haskell Features Missing in Pan#

• User-definable infix operators. An identifier can be converted into an operator but there is no way to assign a precedence other that 9 (left associative) to the operator.
• Sections.
• case expressions.
• Complex numbers.
• General looping. All loops must be statically unrollable.
• Selective import / export in modules.

7.2  Extensions / Incompatibilities

• Connections (binds in let statements): these are a manifestation of the UI monad in the original Pan. Read code such as
    let x <- foo
        y = bar
        z <- blah
     in baz
  as the following:
    do x <- foo
       let y = bar
       z <- blah
       return baz
  Unlike the normal Haskell "do", the monadic binding in Pan# are fully recursive. The main module (but no other) has an implicit "monadic let" in it, allowing binds in the top level definitions.
• Type coercion: Pan# coerces the type a to Framed a where needed. This "implicit return" allows the user to use function such as beside on pictures instead of framed pictures. The MFramed type is really a multi-parameter type class, as follows:
  class MFramed a b | a -> b where
    frameIt :: a -> b
  instance MFramed (Framed a) a where
    frameIt = id
  instance MFramed T T where  -- for all types T except Framed
    frameIt x = return x
  A signature such as
  beside :: MFramed ImageC -> MFramed ImageC -> Framed ImageC
  would be as follows in Haskell:
  beside :: (MFramed a ImageC, MFramed b ImageC) => a -> b -> Framed ImageC
  
• Views: the dual polar / cartesian views of points are not supported in Haskell. This is currently the only view in the system.
• Variables can start with upper case letters.