Hello, World! – using Predefined¶
Here is a simple “Hello, World!” Mathics3 function.
from mathics.builtin.base import Predefined
from mathics.core.atom import String
class Hello(Predefined):
def evaluate(self, evaluation: Evaluation) -> String:
return String(f"Hello, World!")
Add the above at the end to a file in mathics.builtin
like system.py
,
Later on, we will show how to add code without modify the Mathics3 core, but for now we’ll start simple.
Now start mathics from the Mathics3 source tree:
$ python mathics/main.py
Mathics3 6.0.0
...
In[1]:= Hello
Out[1]= Hello, World!
Now let’s go over the code. For a Symbol Hello
we
define a Python Class
of type Predefined
. Predifined
is perhaps the
most primitive class that is used for adding Mathics3 Symbols.
In that class you define a method evaluate(self, evaluation) which
is what will get called when the Symbol is evaluated. The
evaluation
parameter contains the evaluation environment that can
be used to get definitions of variables and other things that may be
needed to perform the function.
However here all we do is return a Mathics3 string, so we don’t need to use what is in evaluation.
The return value of a Mathics3 function should be some sort of
superclass of BaseElement Class. A String
is a subclass of
the Atom Class which in turn is a subclass of the
BaseElement
. You can also return the None
value, in which case
the expression is unchanged.