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Internal Structures

Internal Structures

A native view of expressions

By default, and in order to avoid having to pay marshalling/unmarshalling costs for each argument every time one invokes an internal R function, we represent R values in exactly the same way R does, as a pointer to a SEXPREC structure (defined in R/Rinternals.h). This choice has a downside, however: Haskell’s pattern matching facilities are not immediately available, since only algebraic datatypes can be pattern matched.

HExp is R’s SEXP (or *SEXPREC) structure represented as a (generalized) algebraic datatype. A simplified definition of HExp would go along the lines of:

data HExp
  = Nil                                           -- NILSXP
  | Symbol { ... }                                -- SYMSXP
  | Real { ... }                                  -- REALSXP
  | ...

We define one constructor for each value of the SEXPTYPE enumeration in <RInternals.h>.

For the sake of efficiency, we do not use HExp as the basic datatype that all inline-r generated code expects. That is, we do not use HExp as the universe of R expressions, merely as a view. We introduce the following view function to locally convert to a HExp, given a SEXP from R.

hexp :: SEXP s -> HExp

The fact that this conversion is local is crucial for good performance of the translated code. It means that conversion happens at each use site, and happens against values with a statically known form. Thus we expect that the view function can usually be inlined, and the short-lived HExp values that it creates compiled away by code simplification rules applied by GHC. In this manner, we get the convenience of pattern matching that comes with a bona fide algebraic datatype, but without paying the penalty of allocating long-lived data structures that need to be converted to and from R internals every time we invoke internal R functions or C extension functions.

Using an algebraic datatype for viewing R internal functions further has the advantage that invariants about these structures can readily be checked and enforced, including invariants that R itself does not check for (e.g. that types that are special forms of the list type really do have the right number of elements). The algebraic type statically guarantees that no ill-formed type will ever be constructed on the Haskell side and passed to R.

We also define an inverse of the view function:

unhexp :: HExp -> SEXP

A form indexed native view of expresions

In reality, inline-r defines HExp in a slightly more elaborate way. Most R functions expect their inputs to have certain predetermined forms. For example, the + function expects that its arguments be of some numeric type. A runtime error will occur when this is not the case. Likewise, append expects its first argument to be a vector, and its last argument to be a subscript. These form restrictions are documented in a systematic way in each function’s manual page. While R itself, nor its implementation, make any attempt to enforce these restrictions statically, Haskell’s type system is rich enough to allow us to do so.

For this reason, inline-r allows the SEXP and HExp types to be indexed by the form of the expression. For example, a value which is known to be a real number can be given the type SEXP s R.Real. In general, one does not always know a priori the form of an R expression, but pattern matching on an algebraic view of the expression allows us to “discover” the form at runtime. In inline-r, we define the HExp algebraic view type as a generalized algebraic datatype (GADT). In this way, the body of each branch can be typed under the assumption that the scrutinee matches the pattern in the left hand side of the branch. For example, in the body of a branch with pattern Real x, the type checker can refine the type of the scrutinee to SEXP s R.Real. In inline-r, HExp is defined as follows:

data HExp s (a :: SEXPTYPE) where
  Nil       :: HExp R.Nil
  -- Fields: pname, value, internal.
  Symbol    :: SEXP s R.Char
            -> SEXP s a
            -> Maybe (SEXP s b)
            -> HExp R.Symbol
  Int       :: {-# UNPACK #-} !(Vector.Vector R.Int Int32)
            -> HExp R.Int
  Real      :: {-# UNPACK #-} !(Vector.Vector R.Real Double)
  ...

See the Haddock generated documentation for the Language.R.HExp module for the full definition.

In the above, notice that the Symbol constructor produces a value of type HExp R.Symbol, while the Real constructor produces a value of type HExp R.Real. In other words, the type index reflects the constructor of each variant, which itself is a function of the form of a SEXP. For safety and clarity, we preclude indexing SEXP and HExp with any Haskell type (which are all usually of kind *). We use GHC’s DataKinds extension to introduce a new kind of types, named SEXPTYPE, and limit the possible type indexes to types that have kind SEXPTYPE. Version 7.4 of GHC and later feature the DataKinds extension to permit defining SEXPTYPE as a regular algebraic datatype and then allowing SEXPTYPE to be considered as a kind and the constructors of this type to be considered types of the SEXPTYPE kind, depending on context. See the relevant section in GHC user’s guide for more information.