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1995-03-23
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Article 1688 of comp.sys.handhelds:
Path: en.ecn.purdue.edu!pur-ee!mentor.cc.purdue.edu!purdue!tut.cis.ohio-state.edu!cs.utexas.edu!hp-sdd!hp-pcd!hpcvra!charliep
From: charliep@hpcvra.CV.HP.COM (Charles Patton)
Newsgroups: comp.sys.handhelds
Subject: Re: Undocumented Functions (HP-48SX)
Message-ID: <21580049@hpcvra.CV.HP.COM>
Date: 30 Mar 90 17:19:00 GMT
References: <53655@microsoft.UUCP>
Organization: Hewlett-Packard Co., Corvallis, OR, USA
Lines: 31
The QUOTE and APPLY functions both designed to allow the user additional
control over the evaluation of arguments. The main use for the QUOTE
function is to allow unevaluated names to be passed as arguments as they
are in derivative, for example. If you want to build a Laplace transform
function in user language it would need to use the QUOTE in the input to
the function:
'Laplace(QUOTE(X^2-A*LN(X)),QUOTE(X))'
The APPLY function is useful for creating lazy-evaluation functions and
indefinite streams. Consider, for example, the Infinite Sum Function
ISUM:
<< -> Summand Variable StartingValue
<< Summand Variable StartingValue 2 ->LIST |
Summand Variable StartingValue 1 + 3 ->LIST 'ISUM' APPLY +
>>
>>
This is a recursive definition and without the APPLY to postpone the
continued evaluation, it would never return until it ran out of room.
Instead we have
'ISUM(X^2,X,1)' EVAL => '1+ISUM(X^2,X,2)'
'1+ISUM(X^2,X,2)' EVAL => '1+(4+ISUM(X^2,X,3))' etc.
This kind of behavior is very useful for computing with infinite series
and recusive solutions of ordinary differential equations.
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