[ghc-steering-committee] #621: Linear constraints, recommendation: accept

[Jakob Brünker]

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
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