Errors

Every error cl-astar signals is an astar-error, whose astar-error-message says what went wrong and what to do about it. An argument of the wrong type, such as a negative id or weight scale, or an unknown diagonal mode, is a type-error instead, signalled as the function is called.

Graphs

Message	Meaning
Can't remove a point: the graph has no point 9.	The graph has no point of that id. Every function that takes a point says so, beginning with what it was asked to do.
Can't connect point 1 to point 9: the graph has no point 9.	One of the points to connect, disconnect or find a path between is missing. points-connected-p and has-point-p answer false instead.
Can't connect point 1 to itself.	A connection joins two different points.
Can't reserve room for 1 point: the graph has 2 already.	reserve-points takes at least the number of points the graph has, as Godot's reserve_space.
A point's position is two reals, as (8 8) or #(8.0 8.0), not (1 2 3).	(setf point-position) takes a sequence of two reals.
COMPUTE-COST returned -1 as the cost of moving from point 1 to point 2: a cost is a real of 0 or more.	A method of one's own returned a cost a search cannot use.
ESTIMATE-COST returned NIL as the estimated cost from point 1 to point 2: an estimate is a real.	A method of one's own returned no real.

Grids

Message	Meaning
Can't read whether a cell is solid: the cell (9, 2) is outside the grid's region (0 0 4 4).	The cell is not in the grid. cell-in-bounds-p tells whether one is.
Can't find a path from (0, 0) to (4, 4): the cell (4, 4) is outside the grid's region (0 0 4 4).	An end of the path is not in the grid.
Can't read whether a cell is solid: the grid's region, offset, cell size or cell shape changed since its cells were made. Call UPDATE-GRID first, which makes them anew, none solid.	The grid is dirty: see Grids.
A grid's offset is two reals, as (8 8) or #(8.0 8.0), not (1 2 3).	:offset, :cell-size and their setf writers take a sequence of two reals.
COMPUTE-CELL-COST returned -1 as the cost of moving from the cell (0, 0) to (1, 0): a cost is a real of 0 or more.	A method of one's own returned a cost a search cannot use.
ESTIMATE-CELL-COST returned NIL as the estimated cost from the cell (0, 0) to (3, 3): an estimate is a real.	A method of one's own returned no real.

make-instance of a graph or grid checks its initargs as make-graph and make-grid do: a region, mode, heuristic or flag of the wrong type is a type-error, an offset or cell size that is not two reals an astar-error.