-- The type parameter @b@ is for the type of binders in the expression tree.
data Expr b
= Var Id -- ^ Variables
+
| Lit Literal -- ^ Primitive literals
+
| App (Expr b) (Arg b) -- ^ Applications: note that the argument may be a 'Type'.
--
-- See "CoreSyn#let_app_invariant" for another invariant
+
| Lam b (Expr b) -- ^ Lambda abstraction
+
| Let (Bind b) (Expr b) -- ^ Recursive and non recursive @let@s. Operationally
-- this corresponds to allocating a thunk for the things
-- bound and then executing the sub-expression.
-- the meaning of /lifted/ vs. /unlifted/).
--
-- #let_app_invariant#
- -- The right hand side of of a non-recursive 'Let' _and_ the argument of an 'App',
+ -- The right hand side of of a non-recursive 'Let'
+ -- _and_ the argument of an 'App',
-- /may/ be of unlifted type, but only if the expression
- -- is ok-for-speculation. This means that the let can be floated around
- -- without difficulty. For example, this is OK:
+ -- is ok-for-speculation. This means that the let can be floated
+ -- around without difficulty. For example, this is OK:
--
-- > y::Int# = x +# 1#
--
- -- But this is not, as it may affect termination if the expression is floated out:
+ -- But this is not, as it may affect termination if the
+ -- expression is floated out:
--
-- > y::Int# = fac 4#
--
-- At the moment, the rest of the compiler only deals with type-let
-- in a Let expression, rather than at top level. We may want to revist
-- this choice.
+
| Case (Expr b) b Type [Alt b] -- ^ Case split. Operationally this corresponds to evaluating
-- the scrutinee (expression examined) to weak head normal form
-- and then examining at most one level of resulting constructor (i.e. you
-- and the 'Type' must be that of all the case alternatives
--
-- #case_invariants#
- -- This is one of the more complicated elements of the Core language, and comes
- -- with a number of restrictions:
+ -- This is one of the more complicated elements of the Core language,
+ -- and comes with a number of restrictions:
--
- -- The 'DEFAULT' case alternative must be first in the list, if it occurs at all.
+ -- The 'DEFAULT' case alternative must be first in the list,
+ -- if it occurs at all.
--
-- The remaining cases are in order of increasing
-- tag (for 'DataAlts') or
-- lit (for 'LitAlts').
- -- This makes finding the relevant constructor easy, and makes comparison easier too.
+ -- This makes finding the relevant constructor easy,
+ -- and makes comparison easier too.
--
-- The list of alternatives must be exhaustive. An /exhaustive/ case
-- does not necessarily mention all constructors:
-- Blue -> ... ) ...
-- @
--
- -- The inner case does not need a @Red@ alternative, because @x@ can't be @Red@ at
- -- that program point.
- | Cast (Expr b) Coercion -- ^ Cast an expression to a particular type. This is used to implement @newtype@s
- -- (a @newtype@ constructor or destructor just becomes a 'Cast' in Core) and GADTs.
+ -- The inner case does not need a @Red@ alternative, because @x@
+ -- can't be @Red@ at that program point.
+
+ | Cast (Expr b) Coercion -- ^ Cast an expression to a particular type.
+ -- This is used to implement @newtype@s (a @newtype@ constructor or
+ -- destructor just becomes a 'Cast' in Core) and GADTs.
+
| Note Note (Expr b) -- ^ Notes. These allow general information to be
-- added to expressions in the syntax tree
+
| Type Type -- ^ A type: this should only show up at the top
-- level of an Arg
deriving (Data, Typeable)
--
-- Here, @f@ gets an @OtherCon []@ unfolding.
- | DFunUnfolding DataCon [CoreExpr]
- -- The Unfolding of a DFunId
+ | DFunUnfolding -- The Unfolding of a DFunId
+ -- See Note [DFun unfoldings]
-- df = /\a1..am. \d1..dn. MkD (op1 a1..am d1..dn)
-- (op2 a1..am d1..dn)
- -- where Arity = n, the number of dict args to the dfun
- -- The [CoreExpr] are the superclasses and methods [op1,op2],
+
+ Arity -- Arity = m+n, the *total* number of args
+ -- (unusually, both type and value) to the dfun
+
+ DataCon -- The dictionary data constructor (possibly a newtype datacon)
+
+ [CoreExpr] -- The [CoreExpr] are the superclasses and methods [op1,op2],
-- in positional order.
-- They are usually variables, but can be trivial expressions
-- instead (e.g. a type application).
-- (where there are the right number of arguments.)
| UnfNever -- The RHS is big, so don't inline it
+\end{code}
+
+
+Note [DFun unfoldings]
+~~~~~~~~~~~~~~~~~~~~~~
+The Arity in a DFunUnfolding is total number of args (type and value)
+that the DFun needs to produce a dictionary. That's not necessarily
+related to the ordinary arity of the dfun Id, esp if the class has
+one method, so the dictionary is represented by a newtype. Example
+
+ class C a where { op :: a -> Int }
+ instance C a -> C [a] where op xs = op (head xs)
+
+The instance translates to
+ $dfCList :: forall a. C a => C [a] -- Arity 2!
+ $dfCList = /\a.\d. $copList {a} d |> co
+
+ $copList :: forall a. C a => [a] -> Int -- Arity 2!
+ $copList = /\a.\d.\xs. op {a} d (head xs)
+
+Now we might encounter (op (dfCList {ty} d) a1 a2)
+and we want the (op (dfList {ty} d)) rule to fire, because $dfCList
+has all its arguments, even though its (value) arity is 2. That's
+why we cache the number of expected
+
+
+\begin{code}
-- Constants for the UnfWhen constructor
needSaturated, unSaturatedOk :: Bool
needSaturated = False
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