{- (c) The University of Glasgow 2006 (c) The GRASP/AQUA Project, Glasgow University, 1992-1998 Check for recursive type constructors. -} {-# LANGUAGE CPP #-} module GHC.Core.TyCon.RecWalk ( -- * Recursion breaking RecTcChecker, initRecTc, defaultRecTcMaxBound, setRecTcMaxBound, checkRecTc ) where #include "HsVersions.h" import GHC.Prelude import GHC.Core.TyCon import GHC.Core.TyCon.Env {- ************************************************************************ * * Walking over recursive TyCons * * ************************************************************************ Note [Expanding newtypes and products] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ When expanding a type to expose a data-type constructor, we need to be careful about newtypes, lest we fall into an infinite loop. Here are the key examples: newtype Id x = MkId x newtype Fix f = MkFix (f (Fix f)) newtype T = MkT (T -> T) Type Expansion -------------------------- T T -> T Fix Maybe Maybe (Fix Maybe) Id (Id Int) Int Fix Id NO NO NO Notice that * We can expand T, even though it's recursive. * We can expand Id (Id Int), even though the Id shows up twice at the outer level, because Id is non-recursive So, when expanding, we keep track of when we've seen a recursive newtype at outermost level; and bail out if we see it again. We sometimes want to do the same for product types, so that the strictness analyser doesn't unbox infinitely deeply. More precisely, we keep a *count* of how many times we've seen it. This is to account for data instance T (a,b) = MkT (T a) (T b) Then (#10482) if we have a type like T (Int,(Int,(Int,(Int,Int)))) we can still unbox deeply enough during strictness analysis. We have to treat T as potentially recursive, but it's still good to be able to unwrap multiple layers. The function that manages all this is checkRecTc. -} data RecTcChecker = RC !Int (TyConEnv Int) -- The upper bound, and the number of times -- we have encountered each TyCon -- | Initialise a 'RecTcChecker' with 'defaultRecTcMaxBound'. initRecTc :: RecTcChecker initRecTc = RC defaultRecTcMaxBound emptyTyConEnv -- | The default upper bound (100) for the number of times a 'RecTcChecker' is -- allowed to encounter each 'TyCon'. defaultRecTcMaxBound :: Int defaultRecTcMaxBound = 100 -- Should we have a flag for this? -- | Change the upper bound for the number of times a 'RecTcChecker' is allowed -- to encounter each 'TyCon'. setRecTcMaxBound :: Int -> RecTcChecker -> RecTcChecker setRecTcMaxBound new_bound (RC _old_bound rec_nts) = RC new_bound rec_nts checkRecTc :: RecTcChecker -> TyCon -> Maybe RecTcChecker -- Nothing => Recursion detected -- Just rec_tcs => Keep going checkRecTc (RC bound rec_nts) tc = case lookupTyConEnv rec_nts tc of Just n | n >= bound -> Nothing | otherwise -> Just (RC bound (extendTyConEnv rec_nts tc (n+1))) Nothing -> Just (RC bound (extendTyConEnv rec_nts tc 1))