[boost] Re: Review Request: FC++

From: David Abrahams (dave_at_[hidden])
Date: 2003-10-09 08:44:30

Brian McNamara <lorgon_at_[hidden]> writes:

> On Wed, Oct 08, 2003 at 10:24:05PM -0400, David Abrahams wrote:
>> After some more thought, I have a question:  what's the meaning of:
>> 
>>    // f(x) -> 2*(x+3)
>>    lambda(X)[ 
>>        let[ 
>>            Y == X %plus% 3      // y = x+3
>>          , Fun == multiplies(2) // fun(x) -> 2*x  NOTE PARENS
>>        ].in[
>>           Fun[Y]                // fun(y)
>>        ]
>>    ]
>> 
>> ??
>> 
>> if it's the same as
>> 
>>    // f(x) -> 2*(x+3)
>>    lambda(X)[ 
>>        let[ 
>>            Y == X %plus% 3      // y = x+3
>>          , Fun == multiplies[2] // fun(x) -> 2*x  NOTE BRACKETS
>>        ].in[
>>           Fun[Y]                // fun(y)
>>        ]
>>    ]
>
> It is the same.  More generally, inside a lambda expression,
>
>    f(cppexpr)   =   f[cppexpr]
>
> holds true for all "f" which do not cause side-effects.  (Subject to
> the constraint that "cppexpr" is a C++ expression, and not a lambda
> expression.  When "cppexpr" is a lambda expression--let's now call it
> "lexpr" instead--"f(lexpr)" usually doesn't compile, and when it does,
> it typically means something different than "f[lexpr]".)

Yeah, I was talking about the specific case where you're currying as
in multiplies[2].

-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com