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POGetting a simple Haskell HsLua example to work
 

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  1. POGetting a simple Haskell HsLua example to work
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    <p>The HsLua examples on the <a href="http://www.haskell.org/haskellwiki/HsLua" rel="nofollow">Haskell Wiki</a> are broken (dostring and dofile are not defined). Looks like the API has changed since the examples were written.</p> <p>However I have been trying to modify the examples to match the current API and not getting much success. Here's a program that <em>should</em> really work!</p> <pre><code>main = do l &lt;- Lua.newstate Lua.openlibs l Lua.loadfile l "configfile.lua" [name,pwd] &lt;- Lua.callfunc l "getuserpwd" "mail.google.com" putStrLn name Lua.close l </code></pre> <p>However, this doesn't even compile, giving me this strange error message -</p> <pre><code>No instance for (Lua.StackValue [String]) arising from a use of `Lua.callfunc' Possible fix: add an instance declaration for (Lua.StackValue [String]) In a stmt of a 'do' expression: [name, pwd] &lt;- Lua.callfunc l "getuserpwd" "mail.google.com" In the expression: do { l &lt;- Lua.newstate; Lua.openlibs l; Lua.loadfile l "configfile.lua"; [name, pwd] &lt;- Lua.callfunc l "getuserpwd" "mail.google.com"; .... } In an equation for `main': main = do { l &lt;- Lua.newstate; Lua.openlibs l; Lua.loadfile l "configfile.lua"; .... } </code></pre> <p>While the content of the file <code>configfile.lua</code> probably doesn't matter (because the haskell code doesn't even compile), it is as below (same as on the wiki page) -</p> <pre><code>function getuserpwd (site) local cookies = { ["www.ibm.com"] = {"joe", "secret"} , ["www.sun.com"] = {"hoe", "another secret"} } if cookies[site] then return cookies[site] elseif site:match("[.]google[.]com$") then return {"boss", "boss"} else return { os.getenv("USER") or "God" , os.getenv("PWD") or "dontdisturb" } end end </code></pre> <p>Could someone please provide me with a working example of Haskell->Lua and Lua->Haskell calls?</p> <p><strong>Edit</strong></p> <p>Okay I changed the type of the return value to a String (from the earlier array of String), and that program <em>does</em> compile! However it now fails at runtime. Here's the modified program -</p> <pre><code>main = do l &lt;- Lua.newstate Lua.openlibs l Lua.loadfile l "configfile.lua" Lua.callfunc l "getuserpwd" "mail.google.com" &gt;&gt;= putStrLn Lua.close l </code></pre> <p>And here's configfile.lua -</p> <pre><code>function getuserpwd (site) return "boss" end </code></pre> <p>And the runtime error message is as follows -</p> <pre><code>**** Exception: user error (attempt to call a nil value) </code></pre>
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