Re: [Haskell-cafe] New to Haskell

Thank you very much! On Tue, 18 Dec 2007 12:17:54 +0200, Jules Bean wrote:

Cristian Baboi wrote:

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On Tue, 18 Dec 2007 10:29:43 +0200, Miguel Mitrofanov wrote:

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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

There are no such guarantees. If you write

a = putStrLn "Hello world" main = do {a; a;}

then your putStrLn would be performed twice. IO actions are first-class values, that's a feature, not a bug. What guarantees that by running the main, the string "Hello world" will be printed exactly twice ?

The semantics of IO, and the guarantees of the runtime.

IO specifies that (>>) means "compose two actions to make a larger action which does the first actions, then the second action".

[do {a; a;} is notation for a >> a]

The RTS specifies that the "main" action is performed exactly once.

Is this dependent on the implementation (if I use GHC or Hugs) or is something that the language say ? Aside: I tried something like this in WinHugs: do { xxx<-getLine ; putStrLn xxx } and pressed two keys at once for the "getLine" action. The result I've got was an "infinite" loop !!!

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- the lambda expressions can be written (input) but cannot be printed (output)

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Yes, since two different lambda expressions can denote the same function. I just want the sistem to be able to print one of these expressions ! Its this too much to ask ? I find it very strange that I can write a lambda expresion, but the system cannot.

Haskell doesn't contain a code representation natively. It is not a "homoiconic" language. Just like C, C++, Java, Python, Perl, and Ruby, the compiler/interpreter is free to transform code into some more efficient form for running (including transformation all the way to native code, which is what ghc does) and once it has done so, it retains no information about the "shape of" the source code which yielded the function.

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