Metaprogramming

Introduction

Metaprogramming provides a way to manipulate code as you would data. Eta provides a form of metaprogramming called template metaprogramming based on GHC's TemplateHaskell. Template metaprogramming allows you to instantiate code templates by supplying the appropriate parameters for a given situation.

Example

In the example below, makeLenses is a template function from the lens package that is evaluated at compile-time and generates some code. The function takes a single parameter: a type.

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import Control.Lens

data Person = Person { _me :: String,
                       _age :: Double }

makeLenses ''Person

The code above will simplify to the code below once the template function makeLenses gets run during compilation.

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import Control.Lens

data Person = Person { _me :: String,
                       _age :: Double }

age :: Lens' Person Double
age f (Person x1 x2) = fmap (\x -> Person x1 x) (f x2)
{-# INLINE age #-}

me :: Lens' Person String
me f (Person x1 x2) = fmap (\x -> Person x x2) (f x1)
{-# INLINE me #-}

More Information

In the future, there will be more detailed documentation on the metaprogramming facilities offered by Eta. In the meantime, you can check out this link for a quick introduction to Template Haskell. Most of the code supplied should work the same as it does in Haskell.

Next Section

In the next section, we will discuss the compatibility of Eta's template metaprogramming with Haskell.