[Haskell-cafe] TypeCasting in Haskell

[Christophe Poucet]

Dear all,

I typically use indirect composite for making AST's. It allows me to 
easily make new types with other annotations without having to duplicate 
all elements but only those that actually change. It also allows a whole 
amalgam of other possibilities. Recently I have observed a type error 
which seems to be fixable only by doing a "typecast". What do I call a 
typecast, you may ask? Basically a noop that changes the type. Here 
attached you will find the code that demonstrates this.


> module TypeCast where
>
> data FooBar foo bar = --- Indirect composite
> Foo { unFoo :: foo}
> | Bar { unBar :: bar}
>
> --- Assume some PFoobar (parsed)
> data PFooBar = PF {unPF :: FooBar String String}
> --- Assume some TFoobar (typed)
> data TFooBar = TF {unTF :: FooBar Int String }
>
> -- Merrily we write our conversion, using binding to optimize slightly
> typer :: PFooBar -> TFooBar
> typer pFooBar =
> case unPF pFooBar of
> f at Foo { unFoo = foo} -> -- We only need to change foo
> TF $ f{unFoo = 1}
> b at Bar { unBar = bar} -> -- We don't need to change this string
> TF $ b -- So we just return b
>
> --- Nice little main to make this a full module:
> main :: IO ()
> main = do
> print . typer . PF . Foo $ "Hello"
>
> --- Type error:
> -- TypeCast.hs:19:11:
> -- Couldn't match `Int' against `String'
> -- Expected type: FooBar Int String
> -- Inferred type: FooBar String String
> -- In the second argument of `($)', namely `b'
> -- In a case alternative: (b at Bar {unBar = bar}) -> TF $ b
>
> --- what is the fix? Basically do a noop on b
> --- b at Bar {unBar = bar} ->
> --- TF $ b{unBar = bar}
>
Cheers,
Christophe

-- 
Christophe Poucet
Ph.D. Student
Phone:+32 16 28 87 20
E-mail: Christophe (dot) Poucet (at) imec (dot) be
Website: http://notvincenz.com/  
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