Write You a Haskell Stephen Diehl

Interpreter
Our evaluator will operate directly on the syntax and evaluate the results in into a Value type.
data Value
= VInt Integer
| VBool Bool
| VClosure String Expr TermEnv
e interpreter is set up an Identity monad. Later it will become a more complicated monad, but for
now its quite simple. e value environment will explicitly threaded around, and whenever a closure is
created we simply store a copy of the local environment in the closure.
type TermEnv = Map.Map String Value
type Interpreter t = Identity t
Our logic for evaluation is an extension of the lambda calculus evaluator implemented in previous chapter.
However you might notice quite a few incomplete patterns used throughout evaluation. Fear not
though, the evaluation of our program cannot “go wrong”. Each of these patterns represents a state that
our type system guarantees will never happen. For example, if our program did have not every variable
referenced in scope then it would never reach evaluation to begin with and would be rejected in our type
checker. We are morally correct in using incomplete patterns here!
eval :: TermEnv -> Expr -> Interpreter Value
eval env expr = case expr of
Lit (LInt k) -> return $ VInt k
Lit (LBool k) -> return $ VBool k
Var x -> do
let Just v = Map.lookup x env
return v
Op op a b -> do
VInt a’ do
VClosure x body clo