10.4. Primitive Module

The semantics of External Core rely on the contents and informal semantics of the primitive module ghc-prim:GHC.Prim. Nearly all the primitives are required in order to cover GHC’s implementation of the Haskell98 standard prelude; the only operators that can be completely omitted are those supporting the byte-code interpreter, parallelism, and foreign objects. Some of the concurrency primitives are needed, but can be given degenerate implementations if it desired to target a purely sequential backend (see Section the Non-concurrent Back End section).

In addition to these primitives, a large number of C library functions are required to implement the full standard Prelude, particularly to handle I/O and arithmetic on less usual types.

For a full listing of the names and types of the primitive operators, see the GHC library documentation [ghcprim].

10.4.1. Non-concurrent Back End

The Haskell98 standard prelude doesn’t include any concurrency support, but GHC’s implementation of it relies on the existence of some concurrency primitives. However, it never actually forks multiple threads. Hence, the concurrency primitives can be given degenerate implementations that will work in a non-concurrent setting, as follows:

  • ThreadIdzh can be represented by a singleton type, whose (unique) value is returned by myThreadIdzh.
  • forkzh can just die with an unimplemented message.
  • killThreadzh and yieldzh can also just die unimplemented since in a one-thread world, the only thread a thread can kill is itself, and if a thread yields the program hangs.
  • MVarzh a can be represented by MutVarzh (Maybe a); where a concurrent implementation would block, the sequential implementation can just die with a suitable message (since no other thread exists to unblock it).
  • waitReadzh and waitWritezh can be implemented using a select with no timeout.

10.4.2. Literals

Only the following combination of literal forms and types are permitted:

Literal formTypeDescription
CharzhUnicode character code
characterCharzhUnicode character specified by ASCII character
stringAddrzhAddress of specified C-format string