Mercurial > repos > other > SevenLanguagesInSevenWeeks

annotate 7-Haskell/day3.hs @ 96:f548c8460d15

Find changesets by keywords (author, files, the commit message), revision number or hash, or revset expression.
Add extra note about monadic-ness
author IBBoard <dev@ibboard.co.uk>
date Sun, 30 Jun 2019 15:50:33 +0100
parents 83b4f23b9df8
children 85a5e9a6ef5c
Ignore whitespace changes - Everywhere: Within whitespace: At end of lines:
rev   line source
95
83b4f23b9df8 First successful attempt at a Haskell monad for hashtable lookup
IBBoard <dev@ibboard.co.uk>
parents:
diff changeset
1 -- Monad tutorials:
83b4f23b9df8 First successful attempt at a Haskell monad for hashtable lookup
IBBoard <dev@ibboard.co.uk>
parents:
diff changeset
2 -- * https://wiki.haskell.org/All_About_Monads
83b4f23b9df8 First successful attempt at a Haskell monad for hashtable lookup
IBBoard <dev@ibboard.co.uk>
parents:
diff changeset
3 -- * https://www.haskell.org/tutorial/monads.html
83b4f23b9df8 First successful attempt at a Haskell monad for hashtable lookup
IBBoard <dev@ibboard.co.uk>
parents:
diff changeset
4 --
83b4f23b9df8 First successful attempt at a Haskell monad for hashtable lookup
IBBoard <dev@ibboard.co.uk>
parents:
diff changeset
5 -- Pre-existing monads: https://hackage.haskell.org/package/base-4.12.0.0/docs/Control-Monad.html#g:2
83b4f23b9df8 First successful attempt at a Haskell monad for hashtable lookup
IBBoard <dev@ibboard.co.uk>
parents:
diff changeset
6
83b4f23b9df8 First successful attempt at a Haskell monad for hashtable lookup
IBBoard <dev@ibboard.co.uk>
parents:
diff changeset
7 module Day3 where
83b4f23b9df8 First successful attempt at a Haskell monad for hashtable lookup
IBBoard <dev@ibboard.co.uk>
parents:
diff changeset
8 -- Build a "hash" table (actually a simple lookup in a key-value list, because hashes are hard)
83b4f23b9df8 First successful attempt at a Haskell monad for hashtable lookup
IBBoard <dev@ibboard.co.uk>
parents:
diff changeset
9 hashtable = [
83b4f23b9df8 First successful attempt at a Haskell monad for hashtable lookup
IBBoard <dev@ibboard.co.uk>
parents:
diff changeset
10 ("Phonetic", [
83b4f23b9df8 First successful attempt at a Haskell monad for hashtable lookup
IBBoard <dev@ibboard.co.uk>
parents:
diff changeset
11 ("Latin", [
83b4f23b9df8 First successful attempt at a Haskell monad for hashtable lookup
IBBoard <dev@ibboard.co.uk>
parents:
diff changeset
12 ("alpha", "a"),
83b4f23b9df8 First successful attempt at a Haskell monad for hashtable lookup
IBBoard <dev@ibboard.co.uk>
parents:
diff changeset
13 ("beta", "b"),
83b4f23b9df8 First successful attempt at a Haskell monad for hashtable lookup
IBBoard <dev@ibboard.co.uk>
parents:
diff changeset
14 ("charlie", "c")
83b4f23b9df8 First successful attempt at a Haskell monad for hashtable lookup
IBBoard <dev@ibboard.co.uk>
parents:
diff changeset
15 ]),
83b4f23b9df8 First successful attempt at a Haskell monad for hashtable lookup
IBBoard <dev@ibboard.co.uk>
parents:
diff changeset
16 ("Greek", [
83b4f23b9df8 First successful attempt at a Haskell monad for hashtable lookup
IBBoard <dev@ibboard.co.uk>
parents:
diff changeset
17 ("alpha", "α"),
83b4f23b9df8 First successful attempt at a Haskell monad for hashtable lookup
IBBoard <dev@ibboard.co.uk>
parents:
diff changeset
18 ("beta", "β"),
83b4f23b9df8 First successful attempt at a Haskell monad for hashtable lookup
IBBoard <dev@ibboard.co.uk>
parents:
diff changeset
19 ("gamma", "γ")
83b4f23b9df8 First successful attempt at a Haskell monad for hashtable lookup
IBBoard <dev@ibboard.co.uk>
parents:
diff changeset
20 ])
83b4f23b9df8 First successful attempt at a Haskell monad for hashtable lookup
IBBoard <dev@ibboard.co.uk>
parents:
diff changeset
21 ])
83b4f23b9df8 First successful attempt at a Haskell monad for hashtable lookup
IBBoard <dev@ibboard.co.uk>
parents:
diff changeset
22 ]
83b4f23b9df8 First successful attempt at a Haskell monad for hashtable lookup
IBBoard <dev@ibboard.co.uk>
parents:
diff changeset
23
83b4f23b9df8 First successful attempt at a Haskell monad for hashtable lookup
IBBoard <dev@ibboard.co.uk>
parents:
diff changeset
24 -- Note: We've got to put the key first so that we can call it in-fix and supply the search key
83b4f23b9df8 First successful attempt at a Haskell monad for hashtable lookup
IBBoard <dev@ibboard.co.uk>
parents:
diff changeset
25 -- Note 2: The input value has to be a "Just x" otherwise we can "Couldn't match type 'Maybe' with []"
83b4f23b9df8 First successful attempt at a Haskell monad for hashtable lookup
IBBoard <dev@ibboard.co.uk>
parents:
diff changeset
26 -- Note 3: I can't get anything out of this other than "Just <val>", even if I include ">>= return"
83b4f23b9df8 First successful attempt at a Haskell monad for hashtable lookup
IBBoard <dev@ibboard.co.uk>
parents:
diff changeset
27 -- Example usage: Just hashtable >>= ht_lookup "Phonetic" >>= ht_lookup "Greek" >>= ht_lookup "alpha"
96
f548c8460d15 Add extra note about monadic-ness
IBBoard <dev@ibboard.co.uk>
parents: 95
diff changeset
28 -- This is monadic because "Just hashtable >>= ht_lookup "Phonetic" >>= ht_lookup "foo" >>= ht_lookup "alpha" >>= return" doesn't explode but returns Nothing
95
83b4f23b9df8 First successful attempt at a Haskell monad for hashtable lookup
IBBoard <dev@ibboard.co.uk>
parents:
diff changeset
29 ht_lookup :: String -> [(String, a)] -> Maybe a
83b4f23b9df8 First successful attempt at a Haskell monad for hashtable lookup
IBBoard <dev@ibboard.co.uk>
parents:
diff changeset
30 ht_lookup _ [] = Nothing
83b4f23b9df8 First successful attempt at a Haskell monad for hashtable lookup
IBBoard <dev@ibboard.co.uk>
parents:
diff changeset
31 ht_lookup search_key ((key, value):tail)
83b4f23b9df8 First successful attempt at a Haskell monad for hashtable lookup
IBBoard <dev@ibboard.co.uk>
parents:
diff changeset
32 | search_key == key = Just value
83b4f23b9df8 First successful attempt at a Haskell monad for hashtable lookup
IBBoard <dev@ibboard.co.uk>
parents:
diff changeset
33 | otherwise = ht_lookup search_key tail