Is this compiler compatible with the compiler flavour we're interested in?

For example this checks if the compiler is actually GHC or is another compiler that claims to be compatible with some version of GHC, e.g. GHCJS.

if compilerCompatFlavor GHC compiler then ... else ...

Is this compiler compatible with the compiler flavour we're interested in, and if so what version does it claim to be compatible with.

For example this checks if the compiler is actually GHC-7.x or is another compiler that claims to be compatible with some GHC-7.x version.

case compilerCompatVersion GHC compiler of
  Just (Version (7:_)) -> ...
  _                    -> ...

Some compilers have a notion of a database of available packages. For some there is just one global db of packages, other compilers support a per-user or an arbitrary db specified at some location in the file system. This can be used to build isolated environments of packages, for example to build a collection of related packages without installing them globally.

We typically get packages from several databases, and stack them together. This type lets us be explicit about that stacking. For example typical stacks include:

[GlobalPackageDB]
[GlobalPackageDB, UserPackageDB]
[GlobalPackageDB, SpecificPackageDB "package.conf.inplace"]

Note that the GlobalPackageDB is invariably at the bottom since it contains the rts, base and other special compiler-specific packages.

We are not restricted to using just the above combinations. In particular we can use several custom package dbs and the user package db together.

When it comes to writing, the top most (last) package is used.

Return the package that we should register into. This is the package db at the top of the stack.

Make package paths absolute

Some compilers support optimising. Some have different levels. For compilers that do not the level is just capped to the level they do support.

Some compilers support emitting debug info. Some have different levels. For compilers that do not the level is just capped to the level they do support.

For the given compiler, return the flags for the supported extensions.

For the given compiler, return the extensions it does not support.

Does this compiler support parallel --make mode?

Does this compiler support reexported-modules?

Does this compiler support thinning/renaming on package flags?

Does this compiler have unified IPIDs (so no package keys)

Does this compiler support package keys?

Does this compiler support unit IDs?

Does this compiler support Haskell program coverage?

Does this compiler support profiling?

Does this compiler support Backpack?

Does this compiler's "ar" command supports response file arguments (i.e. @file-style arguments).

Does this compiler's "ar" command support llvm-ar's -L flag, which compels the archiver to add an input archive's members rather than adding the archive itself.

Does this compiler support a package database entry with: "dynamic-library-dirs"?

Does this compiler support a package database entry with: "visibility"?

Does this compiler support the -jsem option?

Some compilers (notably GHC) support profiling and can instrument programs so the system can account costs to different functions. There are different levels of detail that can be used for this accounting. For compilers that do not support this notion or the particular detail levels, this is either ignored or just capped to some similar level they do support.