Hi Micheal,
I am so sorry that I seem to have sent a blank email.
I want to express: for any n,I can get the nth element of x.
val _= type_abbrev("numlist",``: num list``);
val x = ``x: numlist``;
I want to keep x is x, so I add a "^" before x
val n_element = ``!n. EL n ^x``;
and the system prompt "Type inference failure",I don't know why there is such a
mistake. If I delete "!n":
val n_element = ``EL n ^x``;
It's succeed.Maybe I have some wrong understanding about universal quantifier.
Thanks for your reply.
Best,
Liu
-----Original Messages-----
From:[email protected]
Sent Time:2017-04-06 11:42:56 (Thursday)
To: [email protected], [email protected]
Cc:
Subject: Re: [Hol-info] A question about EL in listTheory.
EL is indeed the right constant to use:
> EVAL ``EL 3 [4;5;6;7;8]``;
val it = |- EL 3 [4; 5; 6; 7; 8] = 7: thm
I don’t know what you’re trying to express by writing ``!n. EL n x``. The
English reading of that term would be “for all n, the nth element of x is
true”. (Remember that the ‘!’ is the universal (“for all”) quantifier.)
Note also that writing
val x = ``x:numlist``
does *not* affect the way x is parsed in other terms. In particular, when you
wrote ``!n. EL n x``, the x in that quotation will have been given the type
:bool list. You can see this by setting the show_types reference:
> show_types := true;
val it = () : unit
> val x = ``x : num list``;
val x = ``(x :num list)``: term
> val n_element = ``!n. EL n x``;
val n_element =
``∀(n :num). EL n (x :bool list)``:
term
I hope this helps,
Michael
From: Liu Gengyang <[email protected]>
Date: Thursday, 6 April 2017 at 12:55
To: hol4_QA <[email protected]>
Subject: [Hol-info] A question about EL in listTheory.
Hi,
I want to use EL in listTheory to get the nth element in a list. For example,
val _= type_abbrev("numlist",``: num list``);
val x = ``x: numlist``;
val n_element = ``!n. EL n x``;
type_of n_element;
val it = ``:bool`` : hol_type
I am wondering why the type of n_element is bool rather than num? How can I get
the nth element in a list?
Regards，
Liu------------------------------------------------------------------------------
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