Derived Show and Read

[Simon Peyton-Jones]

Malcolm, Huggers,

Olaf points out that the defn of derived instances of Read
in the H98 appendix isn't right.  In particular, to avoid=20
problems we should require parentheses when the precedence
level doesn't change, even if the associativity could in principle
let us do without. =20

I propose to replace the first set of bullets in D.4 with the
following, and change the example code to match.  (I think I'll make
the constructor infixr, just to make the point.) =20

[This replaces my earlier proposal to change 'less than' to=20
'less than or equal to'.]

This change will affect Hugs and nhc: in particular, 'Show' should
output parentheses even if associativity might allow them to
be omitted.

Before enshrining this change I wanted to check that it looks
reasonable to you.  RSVP

Simon

=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D

"Derived instances of Read make the following assumptions,
which derived instances of Show obey:

1.  If the constructor is defined to be an infix operator, then the
derived Read instance will parse only infix applications of the
constructor (not the prefix form).

2. Associativity is not used to reduce the occurrence of=20
parentheses, although precedence may be. For example, given
	infixr :$
	data T =3D Int :$ Int
then:
  *   "show (1 :$ 2 :$ 3)" produces the string "1 :$ (2 :$ 3)".
  *   (read "1 :$ (2 :$ 3)") succeeds, with the obvious result.
  *   (read "1 :$ 2 :$ 3") fails.


3.  Similarly, if the constructor is defined using record syntax, the
derived Read will parse only the record-syntax form, and furthermore,
the fields must be given in the same order as the original
declaration.

4.  The derived Read instance allows arbitrary Haskell whitespace
between tokens of the input string.  Extra parenthese are also
allowed."


Outside the bullets I'll collect the stuff about Show:

"The result of @show@ is a syntactically correct \Haskell{} expression
containing only constants, given the fixity declarations in force at
the point where the type is declared.  It contains only the
constructor names defined in the data type, parentheses, and spaces.
When labelled constructor fields are used, braces, commas, field
names, and equal signs are also used.  Spaces and parentheses are only
added where needed, ignoring associativity.  No line breaks are added.
The result of @show@ is readable by @read@ if all component types are
readable.  (This is true for all instances defined in the Prelude but
may not be true for user-defined instances.)"