The type checking rules of Tiger, or rather its binding rules, justify
the contrived parsing of declarations.
This is why this section uses
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,
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
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.