Working with GADTs

In this recipe, we will work with GADTs. GADTs extend the data constructors, and allow us more expressivity for representing a complex structure such as a DSL. In this recipe, we will use GADTs to create an expression representation, and a simple parser.

How to do it...

1. Create a new project working-with-GADTs with simple stack template:

        stack new working-with-GADTs simple

2. Open src/Main.hs. We will be adding our source here.
3. Enable GADTs, and StandaloneDeriving:

 {-# LANGUAGE GADTs, StandaloneDeriving #-} moduleMainwhere import Control.Monad import Data.Char import Control.Applicative

4. GADTs take an algebraic data type one step further, and allow us to write data constructors explicitly. For example, we can represent a set of expressions as follows:

 dataExprwhereValue ::Int->ExprAdd ::Expr->Expr->ExprMult ::Expr->Expr->Expr derivinginstanceShowExpr

5. We can evaluate the preceding expression as follows:

 eval ::Expr->Int
 eval (Value i) = i
 eval (Add e1 e2...