[Agda] Re: Erasable vs. irrelevant

Andreas Abel andreas.abel at ifi.lmu.de
Sat Jan 21 10:09:12 CET 2012

Hello Stefan,

On 21.01.12 5:48 AM, Stefan Monnier wrote:
>>> I'm quite confused about these two concepts - as to whether they are
>>> the same or not.
>> They are not the same.  More stuff is erasable than irrelevant.  This is
>> because irrelevance maintains termination of *open* terms, i.e., terms with
>> free variables that can have empty types.
> Is there a place where those terms are precisely defined (at least in
> the context of Agda), BTW?
> In the research literature, it seems that various terms (erasure,
> irrelevance, implicit) are used, and they don't always refer to the
> same thing.

Well, this is an active area of research, concepts are still shifting, 
standard notions have not yet emerged.

I have learned to distinguish whether

* you are concerned with program extraction (use "erasable") or with
   equality of terms with free variables (use "irrelevant/implicit")

* equality is untyped (use "implicit") or typed (use "irrelevant")

This is the world from my perspective only.


Andreas Abel  <><      Du bist der geliebte Mensch.

Theoretical Computer Science, University of Munich
Oettingenstr. 67, D-80538 Munich, GERMANY

andreas.abel at ifi.lmu.de

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