[Haskell-cafe] Re: New to Haskell

18 Dec 2007

      "Cristian Baboi"  writes:
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On Tue, 18 Dec 2007 11:56:36 +0200, Jon Fairbairn
 wrote:
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"Cristian Baboi"  writes:
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- the syntax for a block
Not sure what you mean by "block".
do a <- [1..10]
   b <- [3,4]
   return (a,b)
is an expression... you can write that same expression as
do {a <- [1..10]; b <- [3,4]; return (a,b)} too.
I mean anything that you can put between "{" "}", and between ";"
That's a bit like asking for the syntax of anything you can
put between "(" and ")"; The braces are used for grouping,
and can group different things:  

case 2 of {1 -> 2 ; 2 -> 2}
do {a <- Just 1; return a}
...
Is this ([1 ,2 ,3 ,4]) a tuple or what ?
It has commas in it!
Not in any meaningful sense...
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- what guarantees are made by the LANGUAGE that an IO action
(such as  do  putStrLn "Hello world" ) is not performed
twice
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As has been pointed out, «do putStrLn "Hello world"» is an
expression that you can bind to a variable and use as many
times as you like.
Yes, but that was not the question.
What make you so sure it will be printed the exact number of
times you  intended ?
I don't understand your question at all, then.  How many
times it gets printed depends on how many times the
programme is run, for one thing. Otherwise, it's a matter of
the definition of the semantics of the language.  Evaluation
of a Haskell programme proceeds from evaluation of «main»,
which returns an object of type IO -- a sequence of
Input/Output operatens -- that is "run". IO doesn't happen
when you evaluate an IO action, it happens when the IO
action is run. For example, if you define

f x = seq (putStrLn "foo!") (x+1)

and have 

main = print (f 2)

the «putStrLn "foo!"» is evaluated because seq forces its
first argument, but the only output you get is 3.
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This is a fundamental property of the language.  A lambda
expression is programme and at runtime the system doesn't
know one lambda expression from another (all it can do with
one is apply it to something).
Even C can apply a function variable to an argument (function pointers).
The secret of good language design is not what the language
allows, it's what the language forbids.

-- 
Jón Fairbairn                                 Jon.Fairbairn@cl.cam.ac.uk