The end-of-line layout rules, as described in Source Code Layout and Continuation Lines, mean an end-of-line indicates the end of a function call if the line ends at the end of a complete argument.
You cannot simply break up a function call with many arguments into several lines, because each line becomes a separate expression, almost certainly causing an error. Use the "\" continuation character in this situation:
foo = standardMaterial twosided:on shininess:34 \
diffuseMap:baz \
opacityMap:bar \
bumpMap:boo