Some doubts about the reading of a file in Prolog using a Stream

Question

I have a doubt about how work this simple Prolog Program that read character from a file and put these character into a list.

The code work well and is the following one:

readFile(FileInput, TextList):- open(FileInput, read, Stream),
                                readCharacter(Stream, TextList),
                                close(Stream),
                                !.


readCharacter(Stream,[]):- at_end_of_stream(Stream).    % Exit condition


readCharacter(Stream,[Char|Tail]):- get0(Stream,Char),
                                    readCharacter(Stream,Tail).

When readFile/2 predicate is call happens that: the input passed file is open in read mode and that file is associated with a stream.

Then call the readCharacter/2 predicate that take the Stream associated to my input file (the file to read) and the TextList that is the list where put the file content.

readCharacter/2 predicate it is divided int two cases:

1) An exit condition that use at_end_of_stream(Stream) predicate that succeeds after the last character of the Stream (so the last character in the file) is read. In this case do nothing and the readCharacter predicate end

2) The second case is this rule that add a read character and ad it to my TextList:

readCharacter(Stream,[Char|Tail]):- get0(Stream,Char),
                                    readCharacter(Stream,Tail).

And here I have a doubt: it read a character from the Stream using get0 and put it in the head of the list (as firs element of the list)...so...why the list don't contain the reverse of the file content?

i.e.

I have a file called myFile.txt that contains this text: abc

So readCharacter first read a character from the Stream and put it in the head of the list so I have:

TextList = [a]

Then readCharacter read b character from the Stream and put it in the head of the list si I have:

TextList = [b,a]

Then readCharacter read c character from the Stream and put it in the head of the list si I have:

TextList = [c,b,a]

If so TextList should contain the reverse content of myFile.txt file but it seems to me that TextList contain the same content of myFile.txt file in the same order.

Why? What am I missing? What is wrong in my reasoning?

Unknown Unknown · Accepted Answer · 2013-04-23T15:20:33

readCharacter(Stream,[Char|Tail]):-
    get0(Stream,Char),
    readCharacter(Stream,Tail).

So you have your Stream, you get a character from it and unify it with Char. You have a yet-to-be initialized list in Tail. You then pass Stream (with one character now gone) and Tail to a recursive call of readCharacter. Rinse and repeat until Stream is at end of file.

The point is that this: [Char|Tail] means that Char is the head of the resulting list, or in other words, before everything else in the list.

If you trace this, you will see how Char gets unified with the codes of the characters you have in your file, in the order they are in the file, on calling get0(Char), but disappear as you call readCharacter recursively.

Then, as you go out of the recursion, the character codes you got from get0 magically reappear in the second argument of readCharacter, on the exit from the call in which they were read, each Char before everything in Tail (as should be, according to [Char|Tail].

Basically, what is wrong with your reasoning (here and in other questions) is that you try too hard to impose your expectations on what you see instead of accepting and trying to recognize the logic in what you observe. Prolog, as all other man-made things, follows a certain logic, most of the time.

Some doubts about the reading of a file in Prolog using a Stream

1 Answers