Architecture
GHC is itself written in Haskell (utilizing a technique known as bootstrapping), but the runtime system for Haskell, essential to run programs, is written in C and C--.
GHC's front end - incorporating the lexer, parser and typechecker - is designed to preserve as much information about the source language as possible until after type inference is complete, toward the goal of providing clear error messages to users. After type checking, the Haskell code is desugared into a typed intermediate language known as "Core" (based on System F, extended with let and case expressions). Recently, Core was extended to support generalized algebraic datatypes in its type system, and is now based on an extension to System F known as System FC.
In the tradition of type-directed compilation, GHC's simplifier, or "middle end", where most of the optimizations implemented in GHC are performed, is structured as a series of source-to-source transformations on Core code. The analyses and transformations performed in this compiler stage include demand analysis (a generalization of strictness analysis), application of user-defined rewrite rules (including a set of rules included in GHC's standard libraries that performs foldr/build fusion), unfolding (called "inlining" in more traditional compilers), let-floating, an analysis that determines which function arguments can be unboxed, constructed product result analysis, specialization of overloaded functions, as well as a set of simpler local transformations such as constant folding and beta reduction.
The back end of the compiler transforms Core code into an internal representation of C--, via an intermediate language STG (short for "Spineless Tagless G-machine"). The C-- code can then take one of three routes: it is either printed as C code for compilation with GCC, converted directly into native machine code (the traditional "code generation" phase), or converted to LLVM virtual machine code for compilation with LLVM. In all three cases, the resultant native code is finally linked against the GHC runtime system to produce an executable.
Read more about this topic: Glasgow Haskell Compiler
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