int.icl

   1 implementation module int
   2
   3 import StdEnv
   4
   5 import Data.Either
   6 import Data.Functor
   7 import Data.Maybe
   8 import Data.List
   9 import Control.Applicative
  10 import Control.Monad
  11 import Control.Monad.State
  12 import Control.Monad.Trans
  13
  14 import ast
  15
  16 int :: Expression -> Either [String] Value
  17 int e = evalStateT (eval e) preamble
  18
  19 :: Eval a :== StateT EvalState (Either [String]) a
  20 :: EvalState :== [([Char], Value)]
  21 preamble =
  22         [(['_if'], Func \i->pure (Func \t->pure (Func \e->eval i >>= \(Bool b)->
  23                 if b (eval t) (eval e))))
  24         ,(['_eq'],  binop \(Int i) (Int j)->Bool (i==j))
  25         ,(['_sub'], binop \(Int i) (Int j)->Int (i-j))
  26         ,(['_add'], binop \(Int i) (Int j)->Int (i+j))
  27         ,(['_mul'], binop \(Int i) (Int j)->Int (i*j))
  28         ,(['_div'], binop \(Int i) (Int j)->Int (i/j))
  29         ]
  30 where
  31         binop op = Func \l->pure (Func \r->op <$> eval l <*> eval r)
  32
  33 sub :: [Char] Expression Expression -> Expression
  34 sub ident subst (Let v b rest)
  35         = Let v (sub ident subst b)
  36                 (if (v == ident) rest (sub ident subst rest))
  37 sub ident subst (Var v)
  38         | ident == v = subst
  39 sub ident subst (App e1 e2) = App (sub ident subst e1) (sub ident subst e2)
  40 sub ident subst (Lambda v b)
  41         | ident <> v = Lambda v (sub ident b subst)
  42 sub _ _ x = x
  43
  44 eval :: Expression -> Eval Value
  45 eval (Let ident body rest)
  46         = eval body >>= \v->modify (\vs->[(ident, v):vs]) >>| eval rest
  47 eval (Lit v) = pure v
  48 eval (Var v) = gets (lookup v) >>= maybe (liftT (Left [toString v +++ " not found"])) pure
  49 eval (App e1 e2) = eval e1 >>= \(Func v)->v e2
  50 eval (Lambda a b) = pure (Func \arg->eval (sub a arg b))