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| -rw-r--r-- | README.md | 2 | ||||
| -rw-r--r-- | ch09/9_b_2.hs | 31 |
2 files changed, 32 insertions, 1 deletions
@@ -126,7 +126,7 @@ are prefixed with 'Module_'. | **_8_d_1_** | yes | 212 | | | **_9_a_1_** | yes | 221 | 9. I/O case study: a library for searching the filesystem | | **_9_b_1_** | yes | 228 | | -| 9_b_2 | | | | +| 9_b_2 | yes | | | | 9_b_3 | | | | | 9_b_4 | | | | | **_9_c_1_** | | 232 | | diff --git a/ch09/9_b_2.hs b/ch09/9_b_2.hs new file mode 100644 index 0000000..990dc83 --- /dev/null +++ b/ch09/9_b_2.hs @@ -0,0 +1,31 @@ +-- 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", ...}] |