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| author | Richard Eisenberg <rae@cs.brynmawr.edu> | 2017-07-31 08:29:48 -0400 |
|---|---|---|
| committer | Richard Eisenberg <rae@cs.brynmawr.edu> | 2017-07-31 08:32:28 -0400 |
| commit | c6d4219ae46cddd63aa2b5762efaf99f815009a4 (patch) | |
| tree | 679b01f380a46f1e9aa69af3b8ebd7073cbcaef9 | |
| parent | 49e334c8ea98cd5ecc81cfe10827538182815723 (diff) | |
| download | haskell-c6d4219ae46cddd63aa2b5762efaf99f815009a4.tar.gz | |
Clarify comment about data family arities
as requested in #14045.
[skip ci] comments only
| -rw-r--r-- | compiler/types/FamInstEnv.hs | 5 |
1 files changed, 5 insertions, 0 deletions
diff --git a/compiler/types/FamInstEnv.hs b/compiler/types/FamInstEnv.hs index f40dabe3fd..b9aa43957e 100644 --- a/compiler/types/FamInstEnv.hs +++ b/compiler/types/FamInstEnv.hs @@ -162,6 +162,11 @@ Over-saturation is also possible: see Note [Eta reduction for data families]. Accordingly, the FamInst is never over-saturated. +Why can we allow such flexibility for data families but not for type families? +Because data families can be decomposed -- that is, they are generative and +injective. A Type family is neither and so always must be applied to all its +arguments. + Note [Eta reduction for data families] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Consider this |