| Author |
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< Erlang ~ (noob) defining a constant value. |
| anupamkapoor |
 Posted: Wed Sep 05, 2007 3:34 pm |
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Joined: 05 Sep 2007
Posts: 7
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hi all,
i have just started looking at erlang and kind of experimenting with toy programs.
i am having a slight problem while defining constant values e.g. i would like to say in (some math_constants) file things like:
PI = 3.14159265358979
E = 2.7182818284590
PHI = 1.6180
g = 0.577215664901
etc. how do i say this in erlang ?
for example, here is a trivial area calculation example:
,----
| -module (area).
| -export ([shape_area/1]).
|
| %% compute the area of simple shapes
| PI = 3.14159265.
|
| shape_area({circle, R}) ->
| PI * R * R.
`----
this gives me the following error:
8> c("/home/anupam/source-code/erlang/area", [{outdir, "/home/anupam/source-code/erlang/"}]).
/home/anupam/source-code/erlang/area.erl:5: syntax error before: PI
/home/anupam/source-code/erlang/area.erl:8: variable 'PI' is unbound
error
any help is greatly appreciated !
thanks
anupam
PS: i have defined a macro of sorts for this, but it just feels *too* perverse. |
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| michal |
| Posted: Wed Sep 05, 2007 5:17 pm |
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Posts: 44
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| anupamkapoor |
| Posted: Wed Sep 05, 2007 5:26 pm |
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Joined: 05 Sep 2007
Posts: 7
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michal wrote:
Macro is the way to go.....
ah ok ! i was almost certain that i was missing something with erlang being single-assignment and all.
is there some way to avoid macros at all for something so trivial ?
thanks
anupam |
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| michal |
| Posted: Wed Sep 05, 2007 5:53 pm |
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Joined: 20 Jul 2006
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You could define a function in some module that will take no arguments and just return a constant. I.e. when you call math:pi(). it returns a float.
But why would you like to avoid macros? Maybe term 'macro' sounds like dedicated for some more complicated definitions, but it is widely used for both macros and constants.
Michal |
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http://www.erlang-consulting.com |
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| anupamkapoor |
| Posted: Wed Sep 05, 2007 6:04 pm |
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Joined: 05 Sep 2007
Posts: 7
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michal wrote:
But why would you like to avoid macros? Maybe term 'macro' sounds like dedicated for some more complicated definitions, but it is widely used for both macros and constants.
Michal
well, i am not very conversant with erlang macros yet. are they akin to c pre-processor macros or more like lisp ?
thanks
anupam |
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| KatolaZ |
| Posted: Thu Sep 06, 2007 10:28 am |
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Joined: 04 Jul 2007
Posts: 5
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Macros in Erlang does not have anything to do with macros in Lisp. They don't add additional constructs to the language, as Lisp macros do, but define "shortcuts", as C preprocessor directives. Erlang macros also preserve a C-like syntax (except for the fact that -define(). is used, instead of #define...  ). For any further explanation see the Erlang Reference Manual, section 7.
HND
Enzo |
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| wuji |
| Posted: Thu Aug 30, 2012 7:05 am |
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| dongdongwu |
| Posted: Thu Sep 20, 2012 2:09 am |
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