@@ -78,21 +78,46 @@ If both the dependency and depending packages aren't compilers or other machine-
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7979Finally, if the depending package is a compiler or other machine-code-producing tool, it might need dependencies that run at "emit time". This is for compilers that (regrettably) insist on being built together with their source languages' standard libraries. Assuming build != host != target, a run-time dependency of the standard library cannot be run at the compiler's build time or run time, but only at the run time of code emitted by the compiler.
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81−Putting this all together, that means we have dependencies in the form "host → target", in at most the following six combinations:
81+Putting this all together, that means that we have dependency types of the form "X→ E", which means that the dependency executes on X and emits code for E; each of X and E can be `build`, `host`, or `target`, and E can be `*` to indicate that the dependency is not a compiler-like package.
82+
83+Dependency types describe the relationships that a package has with each of its transitive dependencies. You could think of attaching one or more dependency types to each of the formal parameters at the top of a package's `.nix` file, as well as to all of *their* formal parameters, and so on. Triples like `(foo, bar, baz)`, on the other hand, are a property of an instantiated derivation -- you could would attach a triple `(mips-linux, mips-linux, sparc-solaris)` to a `.drv` file in `/nix/store`.
84+
85+Only nine dependency types matter in practice:
8286
8387#### Possible dependency types {#possible-dependency-types}
8488
85−| Dependency’s host platform | Dependency’s target platform |
86−|----------------------------|------------------------------|
87−| build | build |
88−| build | host |
89−| build | target |
90−| host | host |
91−| host | target |
92−| target | target |
89+| Dependency type | Dependency’s host platform | Dependency’s target platform |
90+|-----------------|----------------------------|------------------------------|
91+| build → * | build | (none) |
92+| build → build | build | build |
93+| build → host | build | host |
94+| build → target | build | target |
95+| host → * | host | (none) |
96+| host → host | host | host |
97+| host → target | host | target |
98+| target → * | target | (none) |
99+| target → target | target | target |
100+
101+Let's use `g++` as an example to make this table clearer. `g++` is a C++ compiler written in C. Suppose we are building `g++` with a `(build, host, target)` platform triple of `(foo, bar, baz)`. This means we are using a `foo`-machine to build a copy of `g++` which will run on a `bar`-machine and emit binaries for the `baz`-machine.
102+
103+* `g++` links against the host platform's `glibc` C library, which is a "host→ *" dependency with a triple of `(bar, bar, *)`. Since it is a library, not a compiler, it has no "target".
104+
105+* Since `g++` is written in C, the `gcc` compiler used to compile it is a "build→ host" dependency of `g++` with a triple of `(foo, foo, bar)`. This compiler runs on the build platform and emits code for the host platform.
106+
107+* `gcc` links against the build platform's `glibc` C library, which is a "build→ *" dependency with a triple of `(foo, foo, *)`. Since it is a library, not a compiler, it has no "target".
108+
109+* This `gcc` is itself compiled by an *earlier* copy of `gcc`. This earlier copy of `gcc` is a "build→ build" dependency of `g++` with a triple of `(foo, foo, foo)`. This "early `gcc`" runs on the build platform and emits code for the build platform.
110+
111+* `g++` is bundled with `libgcc`, which includes a collection of target-machine routines for exception handling and
112+software floating point emulation. `libgcc` would be a "target→ *" dependency with triple `(foo, baz, *)`, because it consists of machine code which gets linked against the output of the compiler that we are building. It is a library, not a compiler, so it has no target of its own.
113+
114+* `libgcc` is written in C and compiled with `gcc`. The `gcc` that compiles it will be a "build→ target" dependency with triple `(foo, foo, baz)`. It gets compiled *and run* at `g++`-build-time (on platform `foo`), but must emit code for the `baz`-platform.
115+
116+* `g++` allows inline assembler code, so it depends on access to a copy of the `gas` assembler. This would be a "host→ target" dependency with triple `(foo, bar, baz)`.
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118+* `g++` (and `gcc`) include a library `libgccjit.so`, which wrap the compiler in a library to create a just-in-time compiler. In nixpkgs, this library is in the `libgccjit` package; if C++ required that programs have access to a JIT, `g++` would need to add a "target→ target" dependency for `libgccjit` with triple `(foo, baz, baz)`. This would ensure that the compiler ships with a copy of `libgccjit` which both executes on and generates code for the `baz`-platform.
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95−Some examples will make this table clearer. Suppose there's some package that is being built with a `(build, host, target)` platform triple of `(foo, bar, baz)`. If it has a build-time library dependency, that would be a "host → build" dependency with a triple of `(foo, foo, *)` (the target platform is irrelevant). If it needs a compiler to be built, that would be a "build → host" dependency with a triple of `(foo, foo, *)` (the target platform is irrelevant). That compiler, would be built with another compiler, also "build → host" dependency, with a triple of `(foo, foo, foo)`.
120+* If `g++` itself linked against `libgccjit.so` (for example, to allow compile-time-evaluated C++ expressions), then the `libgccjit` package used to provide this functionality would be a "host→ host" dependency of `g++`: it is code which runs on the `host` and emits code for execution on the `host`.
96121
97122### Cross packaging cookbook {#ssec-cross-cookbook}
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