[Haskell-cafe] Source code location in IO?

[Erik Hesselink]

It feels weird to me that this program transformation produces
different output, and I can't seem to think of another IO action that
behaves this way. Essentially, you'd want `IO a` to have different
semantics from other `a`s, in that let binding it changes what it
does.

However, you can do this with the `qLocation` Template Haskell
function in Language.Haskell.TH.Syntax. Does that help?

Erik

On 20 June 2016 at 17:03, Tom Ellis
<tom-lists-haskell-cafe-2013 at jaguarpaw.co.uk> wrote:
> Is it compatible with the semantics of Haskell to have a function
>
>     sourceLocation :: IO String
>
> which when run returns the source file location at which it is used?  For
> example, suppose Main.hs is
>
>     module Main where
>
>     main = do
>        putStrLn =<< sourceLocation
>        putStrLn "Hello"
>        putStrLn =<< sourceLocation
>
> It would print the following when run
>
>     Main.hs:4
>     Hello
>     Main.hs:6
>
> and
>
>     module Main where
>
>     main = do
>        let s = sourceLocation
>        putStrLn =<< s
>        putStrLn "Hello"
>        putStrLn =<< s
>
> It would print the following when run
>
>     Main.hs:4
>     Hello
>     Main.hs:4
>
> If this is not compatible with the semantics of Haskell, why not?  I agree
> that the two programs must have the same denotation, but is there anything
> that requires them to have the same output when run?
>
> Tom
>
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