[Haskell-cafe] Fwd: meaning of "referential transparency"

[Alexander Solla]

On Sat, Apr 6, 2013 at 10:43 AM, Henning Thielemann <
lemming at henning-thielemann.de> wrote:

>
> Can someone enlighten me about the origin of the term "referential
> transparency"? I can lookup the definition of "referential transparency" in
> the functional programming sense in the Haskell Wiki and I can lookup the
> meaning of "reference" and "transparency" in a dictionary, but I don't know
> why these words were chosen as name for this defined property.
>

Willard V.O. Quine

Referential transparency arises in constrast to referential opacity.  A
context is referentially opaque (reference can't "see" through the context)
when the substitution of equal values are not necessarily equal.  For
example, quotation marks are referentially opaque:

"It is necessary that four and four is eight"
"The number of planets is eight"

If these contexts were referentially transparent, we would find that

"It is necessary that the number of planets is eight",

 a situation which at the very least puts us in a tight ontological
position.
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