We present
calf
, a
c
ost-
a
ware
l
ogical
f
ramework for studying quantitative aspects of functional programs. Taking inspiration from recent work that reconstructs traditional aspects of programming languages in terms of a modal account of
phase distinctions
, we argue that the cost structure of programs motivates a phase distinction between
intension
and
extension
. Armed with this technology, we contribute a synthetic account of cost structure as a computational effect in which cost-aware programs enjoy an internal noninterference property: input/output behavior cannot depend on cost. As a full-spectrum dependent type theory,
calf
presents a unified language for programming and specification of both cost and behavior that can be integrated smoothly with existing mathematical libraries available in type theoretic proof assistants.
We evaluate
calf
as a general framework for cost analysis by implementing two fundamental techniques for algorithm analysis: the
method of recurrence relations
and
physicist’s method for amortized analysis
. We deploy these techniques on a variety of case studies: we prove a tight, closed bound for Euclid’s algorithm, verify the amortized complexity of batched queues, and derive tight, closed bounds for the sequential and
parallel
complexity of merge sort, all fully mechanized in the Agda proof assistant. Lastly we substantiate the soundness of quantitative reasoning in
calf
by means of a model construction.