[Haskell-cafe] Re: Tutorial uploaded

[Tomasz Zielonka]

On Tue, Dec 20, 2005 at 10:04:16PM +0100, Benjamin Franksen wrote:
> and would also be unsafe without some kind of strong guarantee that each 
> single 'world' value is unique. (This is how they do it in Clean.) 
> Imagine
> 
>   g :: World -> World
>   g world = let world' = putStrLn "first line" world
>             in putStrLn "second line" world -- oops, forgot the "'"

A more scary example:

  getLine :: World -> (World, String)

  getTwoLines :: World -> (World, String, String)
  getTwoLines world =
      let (world', line1) = getLine world
          (world'', line2) = getLine world -- oops, should use world' here
      in  (world'', line1, line2)

So we forgot about world', but used line1, which was produced together
with world'.

Best regards
Tomasz

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