Études for Erlang

Étude 3-2: Guards

Even though you won’t get an error message when calculating the area of a shape that has negative dimensions, it’s still worth guarding your area/3 function. You will want two guards for each pattern to make sure that both dimensions are greater than or equal to zero. Since both have to be non-negative, use commas to separate your guards.

Here is some sample output.

1> c(geom).
{ok,geom}
2> geom:area(rectangle, 3, 4).
12
3> geom:area(ellipse, 2, 3).
18.84955592153876
4> geom:area(triangle, 4, 5).
10.0
5> geom:area(square, -1, 3).
** exception error: no function clause matching geom:area(square,-1,3) (geom.erl, line 18)

See a suggested solution in Appendix A.