Plain source file: base/transforms/BetaReduce.hs (2006-07-06)

BetaReduce is imported by: PfeTransformCmds.

module BetaReduce where
import HasBaseStruct(hsApp,hsLambda,hsEVar,hsParen)
import Substitute(subst,mapExp)
import HsExpStruct
import HsExpUtil(exposeE)
import HsPatUtil(isPVar)
import Maybe(fromMaybe)

Find outermost beta redexes and reduce them. Beta redexes are expressions of the form

(\ x1 ... xn -> e) e1 ... en.

Pattern matching is not implemented.

beta e =
    case isLambda fun of
      Just (ps, body) -> substArgs [] ps args body
      _ -> mapExp beta e
    (fun,args) = flatApp e []

    substArgs s (p:ps) (arg:args) body =
      case isPVar p of
        Just x -> substArgs ((x,arg):s) ps args body
        _ -> keep s (p:ps) (arg:args) body
    substArgs s ps args body = keep s ps args body

    keep s ps args body = pApps (hsLambda' ps (subst f body)) args
      where f y = fromMaybe (hsEVar y) (lookup y s)

    flatApp e args =
      case exposeE e of
        Just (HsApp fun arg) -> flatApp fun (arg:args)
        --Just (HsInfixApp e1 i e2) -> can not be a beta redex
        _ -> (e,args)

    pApps fun [] = fun
    pApps fun args = apps (hsParen fun) args

    apps fun [] = fun
    apps fun (arg:args) = apps (hsApp fun arg) args

isLambda e =
  case exposeE e of
    Just (HsLambda ps body) -> Just (ps,body)
    _ -> Nothing

-- Don't create arity 0 lambda asbtractions.
hsLambda' [] e = e
hsLambda' ps e = hsLambda ps e


(HTML for this module was generated on 2006-08-12. About the conversion tool.)