TC-2 Chunks¶
The type checking rules of Tiger, or rather its binding rules, justify
the contrived parsing of declarations.
This is why this section uses -b/--bindings-compute, implemented
later (see TC-3, Bindings).
In Tiger, to support recursive types and functions, continuous
declarations of functions and continuous declarations of types are
considered “simultaneously”. For instance in the following program,
foo and bar are visible in each other’s scope, and
therefore the following program is correct wrt type checking.
let
function foo() : int = bar()
function bar() : int = foo()
in
0
end
$ tc -b foo-bar.tig
$ echo $?
0
In the following sample, because bar is not declared in the
same bunch of declarations, it is not visible during the declaration
of foo. The program is invalid.
let
function foo() : int = bar()
var stop := 0
function bar() : int = foo()
in
0
end
$ tc -b foo-stop-bar.tig
foo-stop-bar.tig:2.26-30: undeclared function: bar
$ echo $?
4
The same applies to types.
We shall name chunk a continuous series of type or function declaration.
A single name cannot be defined more than once in a chunk.
let
function foo() : int = 0
function bar() : int = 1
function foo() : int = 2
var stop := 0
function bar() : int = 3
in
0
end
$ tc -b fbfsb.tig
fbfsb.tig:4.3-26: redefinition: foo
fbfsb.tig:2.3-26: first definition
$ echo $?
4
It behaves exactly as if chunks were part of embedded let in end, i.e. as if the previous program was syntactic sugar for the following one (in fact, Tiger 2006 used to desugar it that way).
let
function foo() : int = 0
function bar() : int = 1
in
let
function foo() : int = 2
in
let
var stop := 0
in
let
function bar() : int = 3
in
0
end
end
end
end
Given the type checking rules for variables, whose definitions cannot be recursive, chunks of variable declarations are reduced to a single variable.