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{-# LANGUAGE TupleSections #-}
module Interpreter where
import Data.Function
import qualified Data.Map as M
{- VAM 2P, done the lazy way -}
data StrTable =
StrTable Int (M.Map Int String)
data Datum
= Atom Int -- unifies a constant
| Struct (Int, Int) -- unifies a structure with arity
deriving (Show)
data Instr
= U Datum -- something unifiable
| NoGoal -- trivial goal
| Goal -- we start a new goal, set up backtracking etc
| Call -- all seems okay, call the goal
| LastCall -- tail call the goal
deriving (Show)
type Code = [Instr]
type Alts = [(Code, Code)] -- clauses to try and the corresponding goals to restore
type Defs = M.Map (Int, Int) [Code]
data Interp =
Interp
{ defs :: Defs -- global definitions for lookup
, hed :: Code -- current head
, gol :: Code -- current goal
, stk :: [Alts] -- possible heads with stored original goal
}
deriving (Show)
prove :: Code -> Defs -> Either (Interp, String) Bool
prove g ds =
let i0 = Interp ds g [LastCall] [[]]
run (Left x) = x
run (Right x) = run $ proveStep Right Left x
in run (Right i0)
{- this gonna need Either String Bool for errors later -}
proveStep :: (Interp -> a) -> (Either (Interp, String) Bool -> a) -> Interp -> a
proveStep c f i = go i
where
ifail msg = f $ Left (i, msg)
withDef f
| Just d <- defs i M.!? f = ($ d)
| otherwise = const $ ifail $ "no definition: " ++ show f
{- Backtracking -}
backtrack i@Interp {stk = ((s, gs):ss):sss} -- backtrack to next clause, restoring goal
= c i {hed = s, gol = gs, stk = ss : sss}
backtrack i@Interp {stk = []:ss@(_:_):sss} -- no next clause, pop stack and backtrack the caller clause
=
backtrack
i {hed = error "failed hed", gol = error "failed gol", stk = ss : sss}
backtrack i@Interp {stk = [[]]} = f (Right False)
backtrack i@Interp {stk = []:[]:_} = ifail "broken stk" -- this should not happen
{- Unification -}
go i@Interp {hed = (U a:hs), gol = (U b:gs)} -- unify constants
= unify a b
where
uok = c i {hed = hs, gol = gs}
unify (Atom a) (Atom b)
| a == b = uok
unify (Struct a) (Struct b)
| a == b = uok
unify _ _ = backtrack i
{- Resolution -}
go i@Interp {hed = [NoGoal], gol = [LastCall], stk = [_]} = f (Right True) -- final success
go i@Interp { hed = [NoGoal]
, gol = [LastCall]
, stk = _:((Goal:U (Struct f):gs, _):ss):sss
} -- goal succeeded, continue with parent frame
=
withDef f $ \(hs:ohs) ->
c i {hed = hs, gol = gs, stk = (map (, gs) ohs ++ ss) : sss}
go i@Interp { hed = [NoGoal]
, gol = (Call:Goal:U (Struct f):gs)
, stk = ss:sss
} -- next goal
=
withDef f $ \(hs:ohs) ->
c i {hed = hs, gol = gs, stk = (map (, gs) ohs ++ ss) : sss}
go i@Interp {hed = (Goal:U (Struct f):ngs), gol = (Call:gs), stk = ss:sss} -- normal call
=
withDef f $ \(hs:ohs) ->
c
i
{ hed = hs
, gol = ngs
, stk = (map (, ngs) ohs) : ((gs, error "gol no hed") : ss) : sss
}
go i@Interp {hed = (Goal:U (Struct f):ngs), gol = [LastCall], stk = _:sss} -- tail call
=
withDef f $ \(hs:ohs) ->
c i {hed = hs, gol = ngs, stk = map (, ngs) ohs : sss}
go _ = ifail "impossible instruction combo"
|
