9_b_2: Add solution

1 files changed, 31 insertions, 0 deletions

diff --git a/ch09/9_b_2.hs b/ch09/9_b_2.hs
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+-- Using 'id' as a control function, 'traverse id' performs a preorder traversal of
+-- a tree: it returns a parent directory before its children. Write a control
+-- function that makes 'traverse' perform a postorder traversal, in which it
+-- returns children before their parent.
+
+-- ghci> :l ControlledVisit.hs
+-- [1 of 1] Compiling ControlledVisit  ( ControlledVisit.hs, interpreted )
+-- Ok, one module loaded.
+
+-- Output of the following commands is manually shortened and formatted for clarity
+-- ghci> traverse' id "test-9_b_1"
+-- [Info {infoPath = "test-9_b_1", ...},
+--  Info {infoPath = "test-9_b_1/dirC", ...},
+--  Info {infoPath = "test-9_b_1/dirC/F", ...},
+--  Info {infoPath = "test-9_b_1/dirC/E", ...},
+--  Info {infoPath = "test-9_b_1/A", ...},
+--  Info {infoPath = "test-9_b_1/B", ...},
+--  Info {infoPath = "test-9_b_1/dirD", ...},
+--  Info {infoPath = "test-9_b_1/dirD/G", ...},
+--  Info {infoPath = "test-9_b_1/dirD/H", ...}]
+
+-- ghci> traverse' (\list -> (tail list) ++ [(head list)]) "test-9_b_1"
+-- [Info {infoPath = "test-9_b_1/dirC/F", ...},
+--  Info {infoPath = "test-9_b_1/dirC/E", ...},
+--  Info {infoPath = "test-9_b_1/dirC", ...},
+--  Info {infoPath = "test-9_b_1/A", ...},
+--  Info {infoPath = "test-9_b_1/B", ...},
+--  Info {infoPath = "test-9_b_1/dirD/G", ...},
+--  Info {infoPath = "test-9_b_1/dirD/H", ...},
+--  Info {infoPath = "test-9_b_1/dirD", ...},
+--  Info {infoPath = "test-9_b_1", ...}]