arithmetic_expression_type_recap.hs

type Id = String    -- Variable identifier
type Env =          -- Environment of evaluation
  Map Id Int

data OpType         -- Types of operations
  = Add             -- * Addition
  | Mul             -- * Multiplication
  deriving (Show, Eq, Ord)

data ExprR r        -- Open recursive expression type
  = Cst Int         -- * Constant
  | Var Id          -- * Variable
  | Op OpType [r]   -- * Operation
  deriving (Show, Eq, Ord)

-- The type of arithmetic DSL (fixed point of ExprR)
type Expr = Fix ExprR