Dear committee,
Arnaud Spiwack and Jack Hughes propose to introduce linear constraints.
These work analogously to linear functions - as can be seen with the new syntax, which is %1 =>, reflecting the existing %1 ->. The motivation is that these constraints make it possible to design linearly typed APIs that are more convenient to use: Without the linear constraints, tokens would have to be passed manually into each function in these cases.
The proposal also introduces dupable classes, which can be used multiple times even when they appear in a linear context, but cannot be passed to an unrestricted function. This is necessary to make some API designs work, see the proposal for details.
To me, it seems that this proposal or something like it is necessary to unlock the full potential of linear types. The proposal lays out why monadic API designs don't provide the same benefits, and while there are potential future GHC developments that could make using it even more convenient (existential types, strict let improvements; see proposal), I believe it would already be sufficiently useful with today's GHC to be a valuable addition. Thus, I recommend acceptance.
Please read through the proposal and voice your opinions.
Best,
Jakob