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Multiplate

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(An example of how to make Multiplate instaces.)
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== Making a Multiplate instance ==
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The easiest way to understand how to use Multiplate is to look at a simple example.
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<pre>
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> import Data.Generics.Multiplate
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</pre>
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Suppose you defined the follow set of mutually recursive data types for a simple language.
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<pre>
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> data Expr = Con Int
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> | Add Expr Expr
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> | Mul Expr Expr
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> | EVar Var
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> | Let Decl Expr
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> deriving (Eq, Show)
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>
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> data Decl = Var := Expr
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> | Seq Decl Decl
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> deriving (Eq, Show)
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>
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> type Var = String
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</pre>
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The first thing we are going to define is a 'plate' for this language.
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<pre>
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> data Plate f = Plate
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> { expr :: Expr -> f Expr
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> , decl :: Decl -> f Decl
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> }
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</pre>
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A plate is a record type that is parametrized by a functor <code>f</code>. There is one field for each type in the mutually recursive structure we want to write generic functions for. Each field has type <code>A -> f A</code> where <code>A</code> is one of the data types.
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To use the Multiplate library we have to make <code>Plate</code> and instance of the <code>Multiplate</code> class. The instance requires that we write two functions: <code>multiplate</code> and <code>mkPlate</code>. Let's define each of these functions in turn.
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<pre>
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> instance Multiplate Plate where
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</pre>
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We have to write one piece of boilerplate code for <code>multiplate</code>. However, once this is implemented, no further boilerplate code need be written.
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<code>multiplate</code> takes a <code>Plate</code> as a parameter. The idea is that for each expression in our language we will call this a function from this <code>Plate</code> parameter on the children of our expression and then combine the results.
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<pre>
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> multiplate plate = Plate buildExpr buildDecl
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> where
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> buildExpr (Add e1 e2) = Add <$> expr plate e1 <*> expr plate e2
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> buildExpr (Mul e1 e2) = Mul <$> expr plate e1 <*> expr plate e2
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> buildExpr (Let d e) = Let <$> decl plate d <*> expr plate e
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> buildExpr e = pure e
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> buildDecl (v := e) = (:=) <$> pure v <*> expr plate e
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> buildDecl (Seq d1 d2) = Seq <$> decl plate d1 <*> decl plate d2
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</pre>
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Notice that when an expression has no children, as in the case of <code>v</code> in <code>v := e</code>, we simply use <code>pure v</code>.
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<code>pure</code> is used to handle the default case in <code>buildExpr</code>, also have no subexpressions.
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Next we have to define <code>mkPlate</code>. <code>mkPlate</code> is a function that builds a <code>Plate</code> given a generic builder function that produces values of type <code>a -> f a</code>. However these generic builder functions require a bit of help. The need to know what the projection function for the field that they are building is, so we pass that as a parameter to them.
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<pre>
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> mkPlate build = Plate (build expr) (build decl)
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</pre>
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That's it. Now we are ready to use out generic library to process our mutually recursive data structure without using any more boilerplate
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== Generic Programing with Multiplate ==
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Coming Soon.
 
Coming Soon.

Revision as of 22:11, 19 November 2010

1 Making a Multiplate instance

The easiest way to understand how to use Multiplate is to look at a simple example.


> import Data.Generics.Multiplate

Suppose you defined the follow set of mutually recursive data types for a simple language.


> data Expr = Con Int
>           | Add Expr Expr
>           | Mul Expr Expr
>           | EVar Var
>           | Let Decl Expr
>           deriving (Eq, Show)
> 
> data Decl = Var := Expr
>           | Seq Decl Decl
>           deriving (Eq, Show)
> 
> type Var = String

The first thing we are going to define is a 'plate' for this language.


> data Plate f = Plate
>            { expr :: Expr -> f Expr
>            , decl :: Decl -> f Decl
>            }

A plate is a record type that is parametrized by a functor f. There is one field for each type in the mutually recursive structure we want to write generic functions for. Each field has type A -> f A where A is one of the data types.

To use the Multiplate library we have to make Plate and instance of the Multiplate class. The instance requires that we write two functions: multiplate and mkPlate. Let's define each of these functions in turn.


> instance Multiplate Plate where

We have to write one piece of boilerplate code for multiplate. However, once this is implemented, no further boilerplate code need be written. multiplate takes a Plate as a parameter. The idea is that for each expression in our language we will call this a function from this Plate parameter on the children of our expression and then combine the results.


>  multiplate plate = Plate buildExpr buildDecl
>   where
>    buildExpr (Add e1 e2) = Add <$> expr plate e1 <*> expr plate e2
>    buildExpr (Mul e1 e2) = Mul <$> expr plate e1 <*> expr plate e2
>    buildExpr (Let d e) = Let <$> decl plate d <*> expr plate e
>    buildExpr e = pure e
>    buildDecl (v := e) = (:=) <$> pure v <*> expr plate e
>    buildDecl (Seq d1 d2) = Seq <$> decl plate d1 <*> decl plate d2

Notice that when an expression has no children, as in the case of v in v := e, we simply use pure v. pure is used to handle the default case in buildExpr, also have no subexpressions.

Next we have to define mkPlate. mkPlate is a function that builds a Plate given a generic builder function that produces values of type a -> f a. However these generic builder functions require a bit of help. The need to know what the projection function for the field that they are building is, so we pass that as a parameter to them.


>  mkPlate build = Plate (build expr) (build decl)

That's it. Now we are ready to use out generic library to process our mutually recursive data structure without using any more boilerplate

2 Generic Programing with Multiplate

Coming Soon.