# HG changeset patch
# User IBBoard <dev@ibboard.co.uk>
# Date 1561903823 -3600
# Node ID 83b4f23b9df8342b9f4e01d9ce79d0225d802469
# Parent  eb868f089bd1ec40c5bfb4288910b2fbc6f35934
First successful attempt at a Haskell monad for hashtable lookup

diff -r eb868f089bd1 -r 83b4f23b9df8 7-Haskell/day3.hs
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/7-Haskell/day3.hs	Sun Jun 30 15:10:23 2019 +0100
@@ -0,0 +1,32 @@
+-- Monad tutorials:
+--  * https://wiki.haskell.org/All_About_Monads
+--  * https://www.haskell.org/tutorial/monads.html
+--
+-- Pre-existing monads: https://hackage.haskell.org/package/base-4.12.0.0/docs/Control-Monad.html#g:2
+
+module Day3 where
+    -- Build a "hash" table (actually a simple lookup in a key-value list, because hashes are hard)
+    hashtable = [
+            ("Phonetic", [
+                ("Latin", [
+                    ("alpha", "a"),
+                    ("beta", "b"),
+                    ("charlie", "c")
+                ]),
+                ("Greek", [
+                    ("alpha", "α"),
+                    ("beta", "β"),
+                    ("gamma", "γ")
+                ])
+            ])
+        ]
+    
+    -- Note: We've got to put the key first so that we can call it in-fix and supply the search key
+    -- Note 2: The input value has to be a "Just x" otherwise we can "Couldn't match type 'Maybe' with []"
+    -- Note 3: I can't get anything out of this other than "Just <val>", even if I include ">>= return"
+    -- Example usage: Just hashtable >>= ht_lookup "Phonetic" >>= ht_lookup "Greek" >>= ht_lookup "alpha"
+    ht_lookup :: String -> [(String, a)] -> Maybe a
+    ht_lookup _ [] = Nothing
+    ht_lookup search_key ((key, value):tail)
+        | search_key == key = Just value
+        | otherwise = ht_lookup search_key tail
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