Re: scheme/dsssl question

[ht@xxxxxxxxxxxxxxx (Henry S. Thompson)]

Chris and Norm ask for eval:

It's not a bug that this is missing, it's a feature.  Scheme, and a
posteriori DSSSL, are lexically scoped.  Consider the following

(define (dblappl fn var) (fn (eval var) (eval var)))

(let ((foo 3)) (dblappl + 'foo))

There is no definition of eval as such which can exhibit coherent
behaviour in this case.   Some dialects of Scheme support access to
lexical binding tables, in which case eval can be defined as a
TWO-argument function, but this is implementation-dependent and tacky.

The normal strategy to get around this feature is to use thunks,
i.e. procedural closures of no arguments:

(define (dblappl fn thunk) (fn (thunk) (thunk)))

(let ((foo 3)) (dblappl + (lambda () foo)))

The thunk of necessity carries its lexical environment with it,
achieving the encapsulation you need.

Hope this helps

ht
-- 
Henry S. Thompson, Human Communication Research Centre, University of Edinburgh
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