ghc-8.2.2: The GHC API

CoreUtils

Description

Commonly useful utilites for manipulating the Core language

Synopsis

# Constructing expressions

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

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

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

Tests whether we have to use a case rather than let binding for this expression as per the invariants of CoreExpr: see CoreSyn

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

# Taking expressions apart

findDefault :: [(AltCon, [a], b)] -> ([(AltCon, [a], b)], Maybe b) #

Extract the default case alternative

addDefault :: [(AltCon, [a], b)] -> Maybe b -> [(AltCon, [a], b)] #

findAlt :: AltCon -> [(AltCon, a, b)] -> Maybe (AltCon, a, b) #

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

isDefaultAlt :: (AltCon, a, b) -> Bool #

mergeAlts :: [(AltCon, a, b)] -> [(AltCon, a, b)] -> [(AltCon, a, b)] #

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

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 -> [(AltCon, [Var], a)] Alternatives -> ([AltCon], [(AltCon, [Var], a)])

refineDefaultAlt :: [Unique] -> TyCon -> [Type] -> [AltCon] -> [CoreAlt] -> (Bool, [CoreAlt]) #

# Properties of expressions

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

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

Is this expression levity polymorphic? This should be the same as saying (isKindLevPoly . typeKind . exprType) but much faster.

type CheapAppFun = Id -> Arity -> 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 change:

case x of _ -> e

into:

e

and to decide whether it's safe to discard a seq.

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. However this can't happen: see CoreSyn. This invariant states that arguments of unboxed type must be ok-for-speculation (or trivial).

exprOkForSpeculation returns True of an expression that is:

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

It is usually called on arguments of unlifted type, but not always In particular, Simplify.rebuildCase calls it on lifted types when a 'case' is a plain seq. See the example in Note [exprOkForSpeculation: case expressions] below

Precisely, it returns True iff: a) The expression guarantees to terminate, b) soon, c) without causing a write side effect (e.g. writing a mutable variable) d) without throwing a Haskell exception e) without risking an unchecked runtime exception (array out of bounds, divide by zero)

For exprOkForSideEffects the list is the same, but omitting (e).

Note that exprIsHNF implies exprOkForSpeculation exprOkForSpeculation implies exprOkForSideEffects

See Note [PrimOp can_fail and has_side_effects] in PrimOp and Note [Implementation: how can_fail/has_side_effects affect transformations]

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.

exprOkForSpeculation returns True of an expression that is:

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

It is usually called on arguments of unlifted type, but not always In particular, Simplify.rebuildCase calls it on lifted types when a 'case' is a plain seq. See the example in Note [exprOkForSpeculation: case expressions] below

Precisely, it returns True iff: a) The expression guarantees to terminate, b) soon, c) without causing a write side effect (e.g. writing a mutable variable) d) without throwing a Haskell exception e) without risking an unchecked runtime exception (array out of bounds, divide by zero)

For exprOkForSideEffects the list is the same, but omitting (e).

Note that exprIsHNF implies exprOkForSpeculation exprOkForSpeculation implies exprOkForSideEffects

See Note [PrimOp can_fail and has_side_effects] in PrimOp and Note [Implementation: how can_fail/has_side_effects affect transformations]

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.

exprIsBig :: Expr b -> Bool #

Returns True of expressions that are too big to be compared by cheapEqExpr

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

rhsIsStatic :: Platform -> (Name -> Bool) -> (Integer -> CoreExpr) -> CoreExpr -> Bool #

This function is called only on *top-level* right-hand sides. Returns True if the RHS can be allocated statically in the output, with no thunks involved at all.

Can we bind this CoreExpr at the top level?

# 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.

See also exprIsBig

cheapEqExpr' :: (Tickish Id -> Bool) -> Expr b -> Expr b -> Bool #

Cheap expression equality test, can ignore ticks by type.

diffExpr :: Bool -> RnEnv2 -> CoreExpr -> CoreExpr -> [SDoc] #

Finds differences between core expressions, modulo alpha and renaming. Setting top means that the IdInfo of bindings will be checked for differences as well.

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.

# Manipulating data constructors and types

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

If the expression is a Coercion, converts.

applyTypeToArgs :: CoreExpr -> Type -> [CoreExpr] -> Type #

A more efficient version of applyTypeToArg when we have several arguments. The first argument is just for debugging, and gives some context

Determines the type resulting from applying an expression with given type to a given argument expression

dataConRepInstPat :: [Unique] -> DataCon -> [Type] -> ([TyVar], [Id]) #

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.

# Working with ticks

stripTicksTop :: (Tickish Id -> Bool) -> Expr b -> ([Tickish Id], Expr b) #

Strip ticks satisfying a predicate from top of an expression

stripTicksTopE :: (Tickish Id -> Bool) -> Expr b -> Expr b #

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

stripTicksTopT :: (Tickish Id -> Bool) -> Expr b -> [Tickish Id] #

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

stripTicksE :: (Tickish Id -> 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!

# StaticPtr

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

Returns Nothing for every other expression.

# Join points

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