用FPC编写方案解释器：递归数据结构 [英] Writing a Scheme interpreter with FPC: Recursive data structures

问题描述

基本上，这是Pascal（FPC）中递归数据结构的一个问题。正如我想实施一个像SICP第4章一样的一个Scheme解释器，这个问题也可能与Schemers有关。 ：）

S表达式应表示为标记数据。到目前为止，我已经构建了一个变量记录，它表示数字和对。希望代码是可读的和不言自明的：

 程序方案; 
 
 type 
 TTag =（ScmFixnum，ScmPair）; 
 PScmObject = ^ TScmObject; 
 TScmObject = record 
 case ScmObjectTag：TTag of 
 ScmFixnum :( ScmObjectFixnum：integer）; 
 ScmPair：（ScmObjectCar，ScmObjectCdr：PScmObject）; 
结束
 
 var 
 Test1：TScmObject; 
 Test2：TScmObject; 
 Test3：TScmObject; 
 
函数MakeFixnum（x：integer）：TScmObject; 
 var 
 fixnum：TScmObject; 
 begin 
 fixnum.ScmObjectTag：= ScmFixnum; 
 fixnum.ScmObjectFixnum：= x; 
 MakeFixnum：= fixnum; 
结束
 
函数MakePair（car，cdr：PScmObject）：TScmObject; 
 var 
 pair：TScmObject; 
 begin 
 pair.ScmObjectTag：= ScmPair; 
 pair.ScmObjectCar：= car; 
 pair.ScmObjectCdr：= cdr; 
 MakePair：= pair; 
结束
 
 begin 
 Test1：= MakeFixnum（7）; 
 writeln（'Test1，Tag：'，Test1.ScmObjectTag，
'，Content：'，Test1.ScmObjectFixnum）; 
 Test2：= MakeFixnum（9）; 
 writeln（'Test2，Tag：'，Test2.ScmObjectTag，
'，Content：'，Test2.ScmObjectFixnum）; 
 Test3：= MakePair（Test1，Test2）; 
结束。 
  

但是，编译代码会产生如下错误：

  $ fpc scheme.pas 
（...）
编译scheme.pas 
 scheme.pas（43,34）错误：参数号不兼容的类型2：得到TScmObject，预期PScmObject
 scheme.pas（45）致命：有1个错误编译模块，停止
致命：编译中止
  

很明显，函数 MakePair 中有一个错误。但我不明白我究竟做错了什么。任何帮助是赞赏。 ：

解决方案

MakePair 函数定义如下：函数MakePair（car，cdr：PScmObject）：TScmObject; p>

请注意，它接收两个类型为的指针PScmObject 。你可以这样调用：

  MakePair（Test1，Test2）; 
  

但是 Test1 和 Test2 的类型为 TScmObject 。所以传递的实际参数是不兼容的，正如编译器所说。

您需要将指针传递给这些记录：

  MakePair（@ Test1，@ Test2）; 
  

---

从长远来看，您将需要要注意这些记录的生命周期。你需要在堆上分配，没有垃圾收集，我怀疑你会进入一个痛苦的世界，试图跟踪谁拥有记录。也许您可以考虑使用界面引用计数来管理生命周期。

Essentially, this is a question about recursive data structures in Pascal (FPC). As I would like to implement a Scheme interpreter like it is shown in SICP chapter 4, this question may be relevant for Schemers as well. :)

S-expressions shall be represented as tagged data. So far, I have constructed a variant record, which represents numbers and pairs. Hopefully the code is readable and self-explanatory:

program scheme;

type
   TTag = (ScmFixnum, ScmPair);
   PScmObject = ^TScmObject;
   TScmObject = record
      case ScmObjectTag: TTag of
         ScmFixnum: (ScmObjectFixnum: integer);
         ScmPair: (ScmObjectCar, ScmObjectCdr: PScmObject);
      end;

var
   Test1: TScmObject;
   Test2: TScmObject;
   Test3: TScmObject;

function MakeFixnum(x: integer): TScmObject;
var
   fixnum: TScmObject;
begin
   fixnum.ScmObjectTag := ScmFixnum;
   fixnum.ScmObjectFixnum := x;
   MakeFixnum := fixnum;
end;

function MakePair(car, cdr: PScmObject): TScmObject;
var
   pair: TScmObject;
begin
   pair.ScmObjectTag := ScmPair;
   pair.ScmObjectCar := car;
   pair.ScmObjectCdr := cdr;
   MakePair := pair;
end;

begin
   Test1 := MakeFixnum(7);
   writeln('Test1, Tag: ', Test1.ScmObjectTag,
           ', Content: ', Test1.ScmObjectFixnum);
   Test2 := MakeFixnum(9);
   writeln('Test2, Tag: ', Test2.ScmObjectTag,
           ', Content: ', Test2.ScmObjectFixnum);
   Test3 := MakePair(Test1, Test2);
end.

However, compiling the code yields an error as follows:

$ fpc scheme.pas
(...)
Compiling scheme.pas
scheme.pas(43,34) Error: Incompatible type for arg no. 2: Got "TScmObject", expected "PScmObject"
scheme.pas(45) Fatal: There were 1 errors compiling module, stopping
Fatal: Compilation aborted

It is obvious that there is an error in the function MakePair. But I do not understand yet what exactly I am doing wrong. Any help is appreciated. :)

解决方案

The MakePair function is defined like this:

function MakePair(car, cdr: PScmObject): TScmObject;

Note that it receives two pointers of type PScmObject. You then call it like this:

MakePair(Test1, Test2);

But Test1 and Test2 are of type TScmObject. So the actual parameters passed are not compatible, just as the compiler says.

You need to pass pointers to these records instead:

MakePair(@Test1, @Test2);

---

In the longer term you are going to need to be careful about the lifetime of these records. You'll need to allocate on the heap and without garbage collection I suspect that you'll enter a world of pain trying to keep track of who owns the records. Perhaps you could consider using interface reference counting to manage lifetime.

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