GHC.Core.Utils

Commonly useful utilities for manipulating the Core language

Constructing expressions

mkCast :: HasDebugCallStack => CoreExpr -> CoercionR -> CoreExpr

Wrap the given expression in the coercion safely, dropping identity coercions and coalescing nested coercions

mkCastMCo :: CoreExpr -> MCoercionR -> CoreExpr

mkPiMCo :: Var -> MCoercionR -> MCoercionR

mkTick :: CoreTickish -> CoreExpr -> CoreExpr

Wraps the given expression in the source annotation, dropping the annotation if possible.

mkTicks :: [CoreTickish] -> CoreExpr -> CoreExpr

mkTickNoHNF :: CoreTickish -> CoreExpr -> CoreExpr

tickHNFArgs :: CoreTickish -> CoreExpr -> CoreExpr

bindNonRec :: HasDebugCallStack => Id -> CoreExpr -> CoreExpr -> CoreExpr

bindNonRec x r b produces either:

let x = r in b

or:

case r of x { _DEFAULT_ -> b }

depending on whether we have to use a case or let binding for the expression (see needsCaseBinding). It's used by the desugarer to avoid building bindings that give Core Lint a heart attack, although actually the simplifier deals with them perfectly well. See also mkCoreLet

needsCaseBinding :: HasDebugCallStack => Type -> CoreExpr -> Bool

needsCaseBinding tests whether we have to use a case rather than let binding for this expression as per the invariants of CoreExpr: see GHC.Core (needsCaseBinding ty rhs) requires that ty has a well-defined levity, else `typeLevity ty` will fail; but that should be the case because needsCaseBinding is only called once typechecking is complete

needsCaseBindingL :: Levity -> CoreExpr -> Bool

mkAltExpr

Arguments

:: AltCon

Case alternative constructor

-> [CoreBndr]

Things bound by the pattern match

-> [Type]

The type arguments to the case alternative

-> CoreExpr

This guy constructs the value that the scrutinee must have given that you are in one particular branch of a case

mkDefaultCase :: CoreExpr -> Id -> CoreExpr -> CoreExpr

mkSingleAltCase :: CoreExpr -> Id -> AltCon -> [Var] -> CoreExpr -> CoreExpr

Taking expressions apart

findDefault :: [Alt b] -> ([Alt b], Maybe (Expr b))

Extract the default case alternative

addDefault :: [Alt b] -> Maybe (Expr b) -> [Alt b]

findAlt :: AltCon -> [Alt b] -> Maybe (Alt b)

Find the case alternative corresponding to a particular constructor: panics if no such constructor exists

isDefaultAlt :: Alt b -> Bool

mergeAlts :: [Alt a] -> [Alt a] -> [Alt a]

Merge alternatives preserving order; alternatives in the first argument shadow ones in the second

trimConArgs :: AltCon -> [CoreArg] -> [CoreArg]

Given:

case (C a b x y) of
       C b x y -> ...

We want to drop the leading type argument of the scrutinee leaving the arguments to match against the pattern

filterAlts

Arguments

:: TyCon

Type constructor of scrutinee's type (used to prune possibilities)

-> [Type]

And its type arguments

-> [AltCon]

imposs_cons: constructors known to be impossible due to the form of the scrutinee

-> [Alt b]

Alternatives

-> ([AltCon], [Alt b])

combineIdenticalAlts :: [AltCon] -> [CoreAlt] -> (Bool, [AltCon], [CoreAlt])

refineDefaultAlt

Arguments

:: [Unique]

Uniques for constructing new binders

-> Mult

Multiplicity annotation of the case expression

-> TyCon

Type constructor of scrutinee's type

-> [Type]

Type arguments of scrutinee's type

-> [AltCon]

Constructors that cannot match the DEFAULT (if any)

-> [CoreAlt]

-> (Bool, [CoreAlt])

True, if a default alt was replaced with a DataAlt

Refine the default alternative to a DataAlt, if there is a unique way to do so. See Note [Refine DEFAULT case alternatives]

scaleAltsBy :: Mult -> [CoreAlt] -> [CoreAlt]

Properties of expressions

exprType :: HasDebugCallStack => CoreExpr -> Type

Recover the type of a well-typed Core expression. Fails when applied to the actual Type expression as it cannot really be said to have a type

coreAltType :: CoreAlt -> Type

Returns the type of the alternatives right hand side

coreAltsType :: [CoreAlt] -> Type

Returns the type of the first alternative, which should be the same as for all alternatives

mkLamType :: HasDebugCallStack => Var -> Type -> Type

Makes a (->) type or an implicit forall type, depending on whether it is given a type variable or a term variable. This is used, for example, when producing the type of a lambda.

mkLamTypes :: [Var] -> Type -> Type

mkLamType for multiple type or value arguments

mkFunctionType :: HasDebugCallStack => Mult -> Type -> Type -> Type

This one works out the FunTyFlag from the argument type See GHC.Types.Var Note [FunTyFlag]

exprIsTrivial :: CoreExpr -> Bool

getIdFromTrivialExpr :: HasDebugCallStack => CoreExpr -> Id

getIdFromTrivialExpr_maybe :: CoreExpr -> Maybe Id

trivial_expr_fold :: (Id -> r) -> (Literal -> r) -> r -> r -> CoreExpr -> r

The worker function for Note [exprIsTrivial] and Note [getIdFromTrivialExpr] This is meant to have the code of both functions in one place and make it easy to derive custom predicates.

(trivial_expr_fold k_id k_triv k_not_triv e) * returns (k_id x) if e is a variable x (with trivial wrapping) * returns (k_lit x) if e is a trivial literal l (with trivial wrapping) * returns k_triv if e is a literal, type, or coercion (with trivial wrapping) * returns k_not_triv otherwise

where "trivial wrapping" is * Type application or abstraction * Ticks other than tickishIsCode * `case e of {}` an empty case

exprIsDupable :: Platform -> CoreExpr -> Bool

exprIsCheap :: CoreExpr -> Bool

exprIsExpandable :: CoreExpr -> Bool

exprIsCheapX :: CheapAppFun -> CoreExpr -> Bool

type CheapAppFun = Id -> Arity -> Bool

exprIsHNF :: CoreExpr -> Bool

exprIsHNF returns true for expressions that are certainly already evaluated to head normal form. This is used to decide whether it's ok to perform case-to-let for lifted expressions, which changes:

case x of x' { _ -> e }

into:

let x' = x in e

and in so doing makes the binding lazy.

So, it does not treat variables as evaluated, unless they say they are. However, it does treat partial applications and constructor applications as values, even if their arguments are non-trivial, provided the argument type is lifted. For example, both of these are values:

(:) (f x) (map f xs)
map (...redex...)

because seq on such things completes immediately.

For unlifted argument types, we have to be careful:

C (f x :: Int#)

Suppose f x diverges; then C (f x) is not a value. We check for this using needsCaseBinding below

exprOkForSpeculation :: CoreExpr -> Bool

To a first approximation, exprOkForSpeculation returns True of an expression that is:

• Safe to evaluate even if normal order eval might not evaluate the expression at all, and
• Safe not to evaluate even if normal order would do so

More specifically, this means that: * A: Evaluation of the expression reaches weak-head-normal-form, * B: soon, * C: without causing a write side effect (e.g. writing a mutable variable).

In particular, an expression that may * throw a synchronous Haskell exception, or * risk an unchecked runtime exception (e.g. array out of bounds, divide by zero) is not considered OK-for-speculation, as these violate condition A.

For exprOkToDiscard, condition A is weakened to allow expressions that might risk an unchecked runtime exception but must otherwise reach weak-head-normal-form. (Note that exprOkForSpeculation implies exprOkToDiscard)

But in fact both functions are a bit more conservative than the above, in at least the following ways:

• W1: We do not take advantage of already-evaluated lifted variables. As a result, exprIsHNF DOES NOT imply exprOkForSpeculation; if y is a case-binder of lifted type, then exprIsHNF y is True, while exprOkForSpeculation y is False. See Note [exprOkForSpeculation and evaluated variables] for why.
• W2: Read-effects on mutable variables are currently also included. See Note [Classifying primop effects] GHC.Builtin.PrimOps.
• W3: Currently, exprOkForSpeculation always returns False for let-expressions. Lets can be stacked deeply, so we just give up. In any case, the argument of exprOkForSpeculation is usually in a strict context, so any lets will have been floated away.

As an example of the considerations in this test, consider:

let x = case y# +# 1# of { r# -> I# r# }
in E

being translated to:

case y# +# 1# of { r# ->
   let x = I# r#
   in E
}

We can only do this if the y# +# 1# is ok for speculation: it has no side effects, and can't diverge or raise an exception.

See also Note [Classifying primop effects] in GHC.Builtin.PrimOps and Note [Transformations affected by primop effects].

exprOkForSpeculation is used to define Core's let-can-float invariant. (See Note [Core let-can-float invariant] in GHC.Core.) It is therefore frequently called on arguments of unlifted type, especially via needsCaseBinding. But it is sometimes called on expressions of lifted type as well. For example, see Note [Speculative evaluation] in GHC.CoreToStg.Prep.

exprOkToDiscard :: CoreExpr -> Bool

To a first approximation, exprOkForSpeculation returns True of an expression that is:

• Safe to evaluate even if normal order eval might not evaluate the expression at all, and
• Safe not to evaluate even if normal order would do so

More specifically, this means that: * A: Evaluation of the expression reaches weak-head-normal-form, * B: soon, * C: without causing a write side effect (e.g. writing a mutable variable).

In particular, an expression that may * throw a synchronous Haskell exception, or * risk an unchecked runtime exception (e.g. array out of bounds, divide by zero) is not considered OK-for-speculation, as these violate condition A.

For exprOkToDiscard, condition A is weakened to allow expressions that might risk an unchecked runtime exception but must otherwise reach weak-head-normal-form. (Note that exprOkForSpeculation implies exprOkToDiscard)

But in fact both functions are a bit more conservative than the above, in at least the following ways:

• W1: We do not take advantage of already-evaluated lifted variables. As a result, exprIsHNF DOES NOT imply exprOkForSpeculation; if y is a case-binder of lifted type, then exprIsHNF y is True, while exprOkForSpeculation y is False. See Note [exprOkForSpeculation and evaluated variables] for why.
• W2: Read-effects on mutable variables are currently also included. See Note [Classifying primop effects] GHC.Builtin.PrimOps.
• W3: Currently, exprOkForSpeculation always returns False for let-expressions. Lets can be stacked deeply, so we just give up. In any case, the argument of exprOkForSpeculation is usually in a strict context, so any lets will have been floated away.

As an example of the considerations in this test, consider:

let x = case y# +# 1# of { r# -> I# r# }
in E

being translated to:

case y# +# 1# of { r# ->
   let x = I# r#
   in E
}

We can only do this if the y# +# 1# is ok for speculation: it has no side effects, and can't diverge or raise an exception.

See also Note [Classifying primop effects] in GHC.Builtin.PrimOps and Note [Transformations affected by primop effects].

exprOkForSpeculation is used to define Core's let-can-float invariant. (See Note [Core let-can-float invariant] in GHC.Core.) It is therefore frequently called on arguments of unlifted type, especially via needsCaseBinding. But it is sometimes called on expressions of lifted type as well. For example, see Note [Speculative evaluation] in GHC.CoreToStg.Prep.

exprOkForSpecEval :: (Id -> Bool) -> CoreExpr -> Bool

A special version of exprOkForSpeculation used during Note [Speculative evaluation]. When the predicate arg fun_ok returns False for b, then b is never considered ok-for-spec.

exprIsWorkFree :: CoreExpr -> Bool

exprIsConLike :: CoreExpr -> Bool

Similar to exprIsHNF but includes CONLIKE functions as well as data constructors. Conlike arguments are considered interesting by the inliner.

isCheapApp :: CheapAppFun

isExpandableApp :: CheapAppFun

isSaturatedConApp :: CoreExpr -> Bool

exprIsTickedString :: CoreExpr -> Bool

Check if the expression is zero or more Ticks wrapped around a literal string.

exprIsTickedString_maybe :: CoreExpr -> Maybe ByteString

Extract a literal string from an expression that is zero or more Ticks wrapped around a literal string. Returns Nothing if the expression has a different shape. Used to "look through" Ticks in places that need to handle literal strings.

exprIsTopLevelBindable :: CoreExpr -> Type -> Bool

Can we bind this CoreExpr at the top level?

altsAreExhaustive :: [Alt b] -> Bool

etaExpansionTick :: Id -> GenTickish pass -> Bool

Should we look past this tick when eta-expanding the given function?

See Note [Ticks and mandatory eta expansion] Takes the function we are applying as argument.

Equality

cheapEqExpr :: Expr b -> Expr b -> Bool

A cheap equality test which bales out fast! If it returns True the arguments are definitely equal, otherwise, they may or may not be equal.

cheapEqExpr' :: (CoreTickish -> Bool) -> Expr b -> Expr b -> Bool

Cheap expression equality test, can ignore ticks by type.

diffBinds :: Bool -> RnEnv2 -> [(Var, CoreExpr)] -> [(Var, CoreExpr)] -> ([SDoc], RnEnv2)

Finds differences between core bindings, see diffExpr.

The main problem here is that while we expect the binds to have the same order in both lists, this is not guaranteed. To do this properly we'd either have to do some sort of unification or check all possible mappings, which would be seriously expensive. So instead we simply match single bindings as far as we can. This leaves us just with mutually recursive and/or mismatching bindings, which we then speculatively match by ordering them. It's by no means perfect, but gets the job done well enough.

Only used in GHC.Core.Lint.lintAnnots

Manipulating data constructors and types

exprToType :: CoreExpr -> Type

If the expression is a Type, converts. Otherwise, panics. NB: This does not convert Coercion to CoercionTy.

applyTypeToArgs :: HasDebugCallStack => SDoc -> Type -> [CoreExpr] -> Type

Determines the type resulting from applying an expression with given type

dataConRepInstPat :: [Unique] -> Mult -> DataCon -> [Type] -> ([TyCoVar], [Id])

dataConRepFSInstPat :: [FastString] -> [Unique] -> Mult -> DataCon -> [Type] -> ([TyCoVar], [Id])

isEmptyTy :: Type -> Bool

True if the type has no non-bottom elements, e.g. when it is an empty datatype, or a GADT with non-satisfiable type parameters, e.g. Int :~: Bool. See Note [Bottoming expressions]

See Note [No alternatives lint check] for another use of this function.

normSplitTyConApp_maybe :: FamInstEnvs -> Type -> Maybe (TyCon, [Type], Coercion)

If normSplitTyConApp_maybe _ ty = Just (tc, tys, co) then ty |> co = tc tys. It's splitTyConApp_maybe, but looks through coercions via topNormaliseType_maybe. Hence the "norm" prefix.

Working with ticks

stripTicksTop :: (CoreTickish -> Bool) -> Expr b -> ([CoreTickish], Expr b)

Strip ticks satisfying a predicate from top of an expression

stripTicksTopE :: (CoreTickish -> Bool) -> Expr b -> Expr b

Strip ticks satisfying a predicate from top of an expression, returning the remaining expression

stripTicksTopT :: (CoreTickish -> Bool) -> Expr b -> [CoreTickish]

Strip ticks satisfying a predicate from top of an expression, returning the ticks

stripTicksE :: (CoreTickish -> Bool) -> Expr b -> Expr b

Completely strip ticks satisfying a predicate from an expression. Note this is O(n) in the size of the expression!

stripTicksT :: (CoreTickish -> Bool) -> Expr b -> [CoreTickish]

InScopeSet things which work over CoreBinds

mkInScopeSetBndrs :: [CoreBind] -> InScopeSet

extendInScopeSetBind :: InScopeSet -> CoreBind -> InScopeSet

extendInScopeSetBndrs :: InScopeSet -> [CoreBind] -> InScopeSet

StaticPtr

collectMakeStaticArgs :: CoreExpr -> Maybe (CoreExpr, Type, CoreExpr, CoreExpr)

collectMakeStaticArgs (makeStatic t srcLoc e) yields Just (makeStatic, t, srcLoc, e).

Returns Nothing for every other expression.

Join points

isJoinBind :: CoreBind -> Bool

Does this binding bind a join point (or a recursive group of join points)?

Tag inference

mkStrictFieldSeqs :: [(Id, StrictnessMark)] -> CoreExpr -> CoreExpr

shouldStrictifyIdForCbv :: Var -> Bool

Do we expect there to be any benefit if we make this var strict in order for it to get treated as as cbv argument? See Note [Which Ids should be strictified] See Note [CBV Function Ids] for more background.

shouldUseCbvForId :: Var -> Bool

unsafeEqualityProof

isUnsafeEqualityCase :: CoreExpr -> Id -> [CoreAlt] -> Maybe CoreExpr

Dumping stuff

dumpIdInfoOfProgram :: Bool -> (IdInfo -> SDoc) -> CoreProgram -> SDoc