MutableToPure.hs

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{-# LANGUAGE UndecidableInstances #-}

module Feldspar.Compiler.Imperative.FromCore.MutableToPure where



import Control.Monad.RWS

import Language.Syntactic
import Language.Syntactic.Constructs.Binding

import Feldspar.Core.Types
import Feldspar.Core.Interpretation
import Feldspar.Core.Constructs.MutableToPure

import Feldspar.Compiler.Imperative.Frontend hiding (Variable)
import Feldspar.Compiler.Imperative.FromCore.Interpretation



instance ( Compile dom dom
         , Lambda TypeCtx :<: dom
         , Variable TypeCtx :<: dom
         )
      => Compile MutableToPure dom
  where
    compileProgSym WithArray _ loc (marr :* (lam :$ body) :* Nil)
        | Just (_, Variable v0)  <- prjDecorCtx typeCtx marr
        , Just (info, Lambda v1) <- prjDecorCtx typeCtx lam
        = do
            e <- compileExpr marr
            withAlias v1 e $ do
              e <- compileExpr body
              tellProg [copyProg loc e]

    compileProgSym WithArray _ loc (marr :* (lam :$ body) :* Nil)
        | Just (info, Lambda v) <- prjDecorCtx typeCtx lam
        = do
            let ta = argType $ infoType info
            let sa = defaultSize ta
            let var = mkVar (compileTypeRep ta sa) v
            withDecl var $ do
              compileProg var marr
              e <- compileExpr body
              tellProg [copyProg loc e]

    compileProgSym RunMutableArray _ loc (marr :* Nil) = compileProg loc marr

{- NOTES:

The trick of doing a copy at the end, i.e. `tellProg [copyProg loc
e]`, when compiling WithArray is important. It allows us to safely
return the pure array that is passed in as input. This is nice because
it allows us to implement `freezeArray` in terms of `withArray`.
In most cases I expect `withArray` to return a scalar as its final
result and then the copyProg is harmless.
-}