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The analogy is comparing writing programs to buying goods:
writing a program with static types is equivalent to buying
with cash, and writing a program with dynamic types is
equivalent to buying with credit.
The "cost" in either case is mental energy and time. When I
write a program with static types I spend a lot of effort up
front, guided by the type system and compiler, to handle edge
cases and generate a totally correct solution. I have to
fully-understand any libraries I'm using and characterize
the problem in terms of types.
When I write a program with dynamic types, I typically get
something I can run much faster, but then I struggle through
compose/test/debug cycles until the program is eventually
correct. The energy cost here is spread out (potentially
over days or weeks) instead of happening earlier.
The correct program is the thing I am buying in this analogy,
not the currency I'm spending.
@milessabin
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Doesn't Spire count?

@non
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non commented Jan 30, 2014

Well, Spire is an interesting case. I mean, it's a big project, but as a library it's pretty modular. Also, there have been many collaborators (which is something a good type system has made way easier).

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