<html><head></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; "><div>You don't need to create a new type for a String -> Int hashtable, you already get it for free because HashTable is a parameterized type.</div><div><br></div><div>Also, although you are apparently trying to make life simpler for yourself using a HashTable, you are actually making like more complicated because the Data.HashTable implementation can only be worked with inside the IO monad. So what you'd need is something like:</div><div><br></div><div><br></div><div>====================</div><div><br></div><div><div><div>import Prelude hiding (lookup)</div><div>import Data.HashTable</div><div><br></div><div>dummy s = 7</div><div><br></div><div>main = do</div><div><span class="Apple-tab-span" style="white-space:pre"> </span>ht <- new (==) dummy</div><div><span class="Apple-tab-span" style="white-space:pre"> </span>insert ht "Foo" 12</div><div> value <- lookup ht "Foo"</div><div> putStrLn . show $ value</div><div><br></div></div></div><div>====================</div><div><br></div><div>Note that I didn't have to label any of the types; Haskell is smart enough to mostly infer all of them automatically. The main reason to include types is if there is some ambiguity. So for example, if you actually wanted to map Strings to Floats, then you would need to explicitly tell it somewhere that the values it is storing are floats. You could do this by either pinning down explicitly the HashTable type:</div><div><br></div><div><div>====================</div><div><br></div><div><div><div>import Prelude hiding (lookup)</div><div>import Data.HashTable</div><div><br></div><div>dummy s = 7</div><div><br></div><div>main = do</div><div><span class="Apple-tab-span" style="white-space: pre; "> </span>ht <- new (==) dummy :: IO (HashTable String Float)</div><div><span class="Apple-tab-span" style="white-space: pre; "> </span>insert ht "Foo" 12</div><div> value <- lookup ht "Foo"</div><div> putStrLn . show $ value</div><div><br></div></div></div><div>====================</div><div><br></div><div>Or just by pinning down the type of a value that you insert into it:</div><div><br></div><div><div>====================</div><div><br></div><div><div><div>import Prelude hiding (lookup)</div><div>import Data.HashTable</div><div><br></div><div>dummy s = 7</div><div><br></div><div>main = do</div><div><span class="Apple-tab-span" style="white-space: pre; "> </span>ht <- new (==) dummy</div><div><span class="Apple-tab-span" style="white-space: pre; "> </span>insert ht "Foo" (12 :: Float)</div><div> value <- lookup ht "Foo"</div><div> putStrLn . show $ value</div><div><br></div></div></div><div>====================</div><div><br></div><div>Again, the downside though is that you can't work with HashTable outside of the IO monad. If what you want is just a map from strings to values, then you probably are better off using Data.Map:</div><div><br></div><div><div>====================</div><div><br></div><div><div><div><div>import Prelude hiding (lookup)</div><div>import Data.Map</div><div><br></div><div>my_map = empty :: Map String Int</div><div><br></div><div>my_map_after_adding_key = insert "Foo" 12 my_map</div><div><br></div><div>value_associated_with_Foo = lookup "Foo" my_map_after_adding_key</div><div><br></div><div>main = putStrLn . show $ value_associated_with_Foo</div></div><div><br></div></div></div><div>====================</div></div><div><br></div><div>Cheers,</div><div>Greg</div></div></div><br><div><div>On Nov 17, 2009, at 11:16 AM, michael rice wrote:</div><br class="Apple-interchange-newline"><blockquote type="cite"><table cellspacing="0" cellpadding="0" border="0"><tbody><tr><td valign="top" style="font: inherit;">I'm trying to create a hash table. Yeah, I know, don't use hash tables, but I need to create something I'm familiar with, not something I've never worked with before. What's wrong with this code?<br><br>Michael<br><br>====================<br><br>import Prelude hiding (lookup)<br>import Data.HashTable<br><br>data MyHashTable = HashTable String Int<br><br>dummy:: String -> Int<br>dummy s = 7<br><br>ht = MyHashTable.new (==) dummy<br><br>====================<br><br>[michael@localhost ~]$ ghci hash1<br>GHCi, version 6.10.3: <a href="http://www.haskell.org/ghc/">http://www.haskell.org/ghc/</a> :? for help<br>Loading package ghc-prim ... linking ... done.<br>Loading package integer ... linking ... done.<br>Loading package base ... linking ... done.<br>[1 of 1] Compiling Main ( hash1.hs, interpreted )<br><br>hash1.hs:9:5: Not in
scope: `MyHashTable.new'<br>Failed, modules loaded: none.<br>Prelude> <br><br></td></tr></tbody></table><br>
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