Sometimes, we know a more precise type than the one the transtyping algorithm can derive. For example if x is a real number, its logarithm might be complex. However, if we are sure x is positive, the logarithm will be real.
To force an expression to belong to type type, use
the syntax:
expr:type
GP2C will check types consistency and output warnings if necessary.
For example
f(x:int)=local(r:real); r=log(x^2+1)
gp2c will complain that the logarithm might not be real. Since x^2+1
is always positive, we can write:
f(x:int)=local(r:real); r=log(x^2+1):real