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| author | Paul Oliver <contact@pauloliver.dev> | 2025-01-03 11:01:20 -0800 |
|---|---|---|
| committer | Paul Oliver <contact@pauloliver.dev> | 2025-01-05 09:59:10 -0800 |
| commit | 6a0d7f5c434c3564d0119befb6799fd77581050a (patch) | |
| tree | f20bc998290211d2a895523417ad32e297b31af0 /ch07_07.1-iii.hs | |
Diffstat (limited to 'ch07_07.1-iii.hs')
| -rw-r--r-- | ch07_07.1-iii.hs | 11 |
1 files changed, 11 insertions, 0 deletions
diff --git a/ch07_07.1-iii.hs b/ch07_07.1-iii.hs new file mode 100644 index 0000000..1285fd9 --- /dev/null +++ b/ch07_07.1-iii.hs @@ -0,0 +1,11 @@ +-- Exercise 7.1-iii +-- Write the `Show` instance for `HasShow` in terms of `elimHasShow`. +data HasShow where + HasShow :: Show t => t -> HasShow + +elimHasShow :: (forall a. Show a => a -> r) -> HasShow -> r +elimHasShow f (HasShow a) = f a + +-- This version also prepends the `HasShow` name to the result of `show`: +instance Show HasShow where + show = mappend "HasShow " . elimHasShow show |