Changes between Version 7 and Version 8 of Records/TypePunningDeclaredOverloadedRecordFields
 Timestamp:
 Mar 12, 2012 4:37:20 AM (4 years ago)
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Records/TypePunningDeclaredOverloadedRecordFields
v7 v8 126 126 127 127 {{{ 128 data HR = HR { rev :: (forall a. [a] > [a]) }  would generate: 129 130 type instance SetResult HR t = HR  HR is not parametric 131 132 newtype Rev = Rev (forall a. [a] > [a]) 133 rev :: HR > Rev > (forall a. [a] > [a])  generated selector is monomorphic 134 rev r = get r (undefined :: Rev) 135 136 instance Has HR Rev where  Has instance generated 137 get HR{ rev = (Rev x) } = x  looks OK, but see GetResult 138 set x HR{ .. } = HR{ rev = x, .. }  note x is already wrapped 139 140 type instance GetResult HR Rev = (forall a. [a] > [a])  no can do! not allowed forall's on RHS 141  (and can't do equality constraints on type instances) 142 143 { instead, do explicit newtype wrapping for higherrank types } 144 128 145 newtype Rev = Rev (forall a. [a] > [a]) deriving (Has) 129 newtype OrdRev = OrdRev (Ord a => [a] > [a]) deriving (Has) 146 newtype OrdRev = OrdRev (Ord a => [a] > [a]) deriving (Has)  class constrained 130 147 131 data HR = HR{ rev :: Rev, ordRev :: OrdRev } sharing (Rev, OrdRev)148 data HRW = HRW{ rev :: Rev, ordRev :: OrdRev } sharing (Rev, OrdRev) 132 149 }}} 133 * Note that to get/set those hr fields needs them to be un/wrapped within the user code. 150 * Now we can now apply the wrapped function polymorphically (after unwrapping within the user code. 151 134 152 135 153 .