<div dir="ltr">Hello all,<div>My problem is the following: I have my own data types, and I'd like to derive automatically instances of some type class from them. I've started looking at GHC.Generics, which offer tools to do exactly that. However, some functions of my typeclass do not take my data type as a parameter, but as a result. Basically:</div>
<div> class MyClass where</div><div> fromString :: String -> a</div><div><br></div><div> data MyData=MkMyData { </div><div style> myField ::Int </div><div style> } deriving (Generic)</div><div style><br></div><div style>
and I want to automatically generate the instance instance MyClass MyData, using default methods, etc.</div><div style>The GHC Generic class does say that it uses a from function that convert from the datatype to its representation: <a name="v:from" class="" style="font-size:13px;margin:0px;padding:0px;font-weight:bold;color:rgb(0,0,0);font-family:monospace;line-height:16px">from</a><span style="font-size:13px;color:rgb(0,0,0);font-family:monospace;line-height:16px;background-color:rgb(240,240,240)"> :: a -> </span><a href="http://www.haskell.org/ghc/docs/7.4.1/html/libraries/ghc-prim-0.2.0.0/GHC-Generics.html#t:Rep" style="font-size:13px;margin:0px;padding:0px;text-decoration:none;color:rgb(171,105,84);font-family:monospace;line-height:16px">Rep</a><span style="font-size:13px;color:rgb(0,0,0);font-family:monospace;line-height:16px;background-color:rgb(240,240,240)"> a x</span><a name="v:from" class="" style="font-size:13px;margin:0px;padding:0px;font-weight:bold;color:rgb(0,0,0);font-family:monospace;line-height:16px">from</a><span style="font-size:13px;color:rgb(0,0,0);font-family:monospace;line-height:16px;background-color:rgb(240,240,240)"> :: a -> </span><a href="http://www.haskell.org/ghc/docs/7.4.1/html/libraries/ghc-prim-0.2.0.0/GHC-Generics.html#t:Rep" style="font-size:13px;margin:0px;padding:0px;text-decoration:none;color:rgb(171,105,84);font-family:monospace;line-height:16px">Rep</a><span style="font-size:13px;color:rgb(0,0,0);font-family:monospace;line-height:16px;background-color:rgb(240,240,240)"> a x</span></div>
<div style>But I don't have a "a" to start from! I see from the related papers that the automatically generated code from from actually does pattern matches on constructors, so I need a value, undefined won't work. However I see the GHC.Generics also provide :+: (<span style="color:rgb(0,0,0);font-family:sans-serif;font-size:13px;line-height:18px">Sums: encode choice between constructors). If I have to provide an value, then the choice between constructor has been done! The examples about generics on </span> <a href="http://www.haskell.org/haskellwiki/GHC.Generics">http://www.haskell.org/haskellwiki/GHC.Generics</a> do provide an example of defining the instance for :+: but I don't understand how we can get there. If I have a class method that takes a value as a parameter, and I pass undefined to it, the code will crash, since it can't pattern match on undefined.</div>
<div style><br></div><div style>Can somebody shed some light on this? Am I using the wrong tool for the job? How can I achieve what I want? I want the full type representation with sums, but without a value to start from.</div>
<div style><br></div><div style>Thanks a million!</div><div><div><br></div><div style>JP</div>-- <br>JP Moresmau<br><a href="http://jpmoresmau.blogspot.com/">http://jpmoresmau.blogspot.com/</a>
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