--> and <--

The --> symbol denotes the forward direction of a chess game with line.

It is used there to mean that the filter should look at future moves from the current position:

line --> check --> check --> [Pp]==0 

will match positions in which the current position is check, the next position is also check, and the position following that has no pawns

The <-- symbol denotes the backwards direction of a chess game in <-- and <-- filters.

<-- is used with the line filter to mean that the filter should look at previous moves from the current position:

line <-- check <-- check <-- [Pp]==0 

will match positions in which the the current position is check, the previous position is also check, and the position prior to that one has no pawns.

The <--symbol is also used with find to denote that the find filter should search backwards:

find <-- move promote [RBN]

will match the currentposition if there is an underpromotion from the current position or an earlier position.

Without the <-- however, the find filter would match the current position if there were an underpromotion from the current position or a later position.

Note: when typing either <-- or --> be sure to use two - characters and not one. CQL will not recognize <- or -> .

Examples

The --> is used extensively in the CQL examples when line is used (see the documentation for line for more examples).

The <-- is used in the white-try.cql example where

 line <-- variation+
      <-- {mainline wtm}

is used to find positions that are descendants of a position in which white makes a move to leave the mainline.