#2104: Add Labels
--------------------------------+-------------------------------------------
Reporter: barney | Owner:
Type: feature request | Status: new
Priority: normal | Component: Compiler
Version: 6.8.2 | Severity: normal
Keywords: | Testcase:
Architecture: Unknown | Os: Unknown
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This is an alternative to #1894 to support the implementation of
ExtensibleRecords. This version is more complex, but both more orthogonal
to the rest of the language, and supports better record features.
1. Add a new syntactic class[[BR]]
label -> ( {{{'}}} (small | large | digit) {small | large | digit |
{{{'}}} }),,<char>,,
A label {{{'name}}} is interpreted as a conid at the value level, and as a
tycon at the type level, with an implicit declaration:
{{{
data 'name = 'name
}}}
but with global scope, so the same label in different modules is the same
value/type.
2. Add a built-in one-parameter type class {{{Label}}} with no methods,
and for each label {{{'name}}} an implicit instance
{{{
instance Label 'name where
}}}
No other instances of {{{Label}}} are allowed.
3. Add a built-in type synonym family {{{LabelCMP}}} of kind {{{* -> * ->
*}}}. For every pair of labels {{{'name1}}} and {{{'name2}}}, one of the
instances
{{{
LabelCMP 'name1 'name2 = LabelLT
LabelCMP 'name1 'name2 = LabelEQ
LabelCMP 'name1 'name2 = LabelGT
}}}
is implicitly defined, depending on the lexicographic order of
{{{'name1}}} and {{{'name2}}}. No other instances of {{{LabelCMP}}} are
allowed. (The types {{{LabelLT}}}, {{{LabelEQ}}} and {{{LabelGT}}} are
datatypes with no constructors, which can be declared in a library.)
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2104>
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