Probably the most important phase in the frontend is the type checker,
which is located at fptools/ghc/compiler/typecheck/.
During type checking, GHC maintains a type environment whose
details are fixed in TcEnv.lhs.
Among other things, the environment contains all imported and local
instances as well as a list of global entities (imported and
local types and classes together with imported identifiers) and
local entities (locally defined identifiers). This environment
is threaded through the type checking monad.
Expressions are type checked by TcExpr.lhs.
Usage occurences of identifiers are processed by the function
tcId whose main purpose is to instantiate
overloaded identifiers. It essentially calls
TcInst.instOverloadedFun once for each universally
quantified set of type constraints. It should be noted that overloaded
identifiers are replaced by new names that are first defined in the LIE
(Local Instance Environment?) and later promoted into top-level
bindings.
GHC implements overloading using so-called dictionaries. A dictionary is a tuple of functions -- one function for each method in the class of which the dictionary implements an instance. During type checking, GHC replaces each type constraint of a function with one additional argument. At runtime, the extended function gets passed a matching class dictionary by way of these additional arguments. Whenever the function needs to call a method of such a class, it simply extracts it from the dictionary.
This sounds simple enough; however, the actual implementation is a bit
more tricky as it wants to keep track of all the instances at which
overloaded functions are used in a module. This information is useful
to optimise the code. The implementation is the module Inst.lhs.
The function instOverloadedFun is invoked for each
overloaded usage occurence of an identifier, where overloaded means that
the type of the idendifier contains a non-trivial type constraint. It
proceeds in two steps: (1) Allocation of a method instance
(newMethodWithGivenTy) and (2) instantiation of functional
dependencies. The former implies allocating a new unique identifier,
which replaces the original (overloaded) identifier at the currently
type-checked usage occurrence.
The new identifier (after being threaded through the LIE) eventually
will be bound by a top-level binding whose rhs contains a partial
application of the original overloaded identifier. This papp applies
the overloaded function to the dictionaries needed for the current
instance. In GHC lingo, this is called a method. Before
becoming a top-level binding, the method is first represented as a value
of type Inst.Inst, which makes it easy to fold multiple
instances of the same identifier at the same types into one global
definition. (And probably other things, too, which I haven't
investigated yet.)
Note: As of 13 January 2001 (wrt. to the code in the
CVS HEAD), the above mechanism interferes badly with RULES pragmas
defined over overloaded functions. During instantiation, a new name is
created for an overloaded function partially applied to the dictionaries
needed in a usage position of that function. As the rewrite rule,
however, mentions the original overloaded name, it won't fire anymore
-- unless later phases remove the intermediate definition again. The
latest CVS version of GHC has an option
-fno-method-sharing, which avoids sharing instantiation
stubs. This is usually/often/sometimes sufficient to make the rules
fire again.
Last modified: Wed Aug 8 19:24:09 EST 2001