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Diffstat (limited to 'ch15_15.4-i.hs')
| -rw-r--r-- | ch15_15.4-i.hs | 10 |
1 files changed, 10 insertions, 0 deletions
diff --git a/ch15_15.4-i.hs b/ch15_15.4-i.hs new file mode 100644 index 0000000..273753d --- /dev/null +++ b/ch15_15.4-i.hs @@ -0,0 +1,10 @@ +-- Exercise 15.4-i +-- Give instances of `SDecide` for `Maybe`. + +-- This is slightly different to Sandy's version but I feel it should work the +-- same. I can't easily test if it compiles though. Importing +-- `Data.Singletons.Decide` pulls in the already existing instances! +instance SDecide a => SDecide (Maybe a) where + SJust a %~ SJust b = a %~ b + SNothing %~ SNothing = Proved Refl + _ %~ _ = Disproved $ const undefined |