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Subject: Re: [boost] [GSoC] [Boost.Hana] Formal review request
From: Louis Dionne (ldionne.2_at_[hidden])
Date: 2014-07-30 15:54:38

Larry Evans <cppljevans <at>> writes:

> [...]
> The correspondence between the two, IIUC, is the let-expresion:
> ( let
> ( [id1 val-expr1]
> [id2 val-expr2]
> [id3 val-expr3]
> ...
> )
> body-expr
> )
> would be, using generic-lambdas:
> []
> ( auto id1
> , auto id2
> , auto id3
> )
> { return body-expr
> }
> ( val-expr1
> , val-expr2
> , val-expr3
> ...
> )
> Is that about right?

That would be it, but you could also use the lambda capture. If I remember,
my motivation for let expressions was to define branches inline when I had
conditionals. Here's what I do in Hana when I want to branch on a (possibly
compile-time) condition:

    auto result = eval_if(condition,
        [](auto _) { return then_branch; },
        [](auto _) { return else_branch; }

Now, I can use the lambda capture if there's something I need to capture in
either branches. I'll explain the dummy argument now. Since we use lambdas,
we achieve one level of laziness; we achieve runtime laziness. Only the branch
which is chosen by the condition will be executed inside eval_if, so all is
good. However, since we also want to support heterogeneous branches, and so
branches whose well-formedness might depend on the value of the condition, we
use a dummy argument to delay the template instantiation inside the branches.
Let's pretend our branches are functions of some arbitrary x:
    auto result = eval_if(condition,
        [](auto _) { return then_branch(_(x)); },
        [](auto _) { return else_branch(_(x)); }

Now, because the compiler does not know what is _ until either branch is
called, it can't instantiate a (possibly invalid) branch. Then, the trick
we play to the compiler is that eval_if always calls us with an identity
function, so _(x) is actually x, but not knowing it until the last moment
bound the compiler to wait before instantiating the whole expression.

This is long, but I hope it clarifies why, IMO, we don't need let
expressions anymore.


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