Interpreting the strictness annotations output by ghc --show-iface

[Edward Z. Yang]

This is the important bit of code in the file:

    instance Outputable Demand where
        ppr Top  = char 'T'
        ppr Abs  = char 'A'
        ppr Bot  = char 'B'

        ppr (Defer ds)      = char 'D' <> ppr ds
        ppr (Eval ds)       = char 'U' <> ppr ds

        ppr (Box (Eval ds)) = char 'S' <> ppr ds
        ppr (Box Abs)       = char 'L'
        ppr (Box Bot)       = char 'X'
        ppr d@(Box _)       = pprPanic "ppr: Bad boxed demand" (ppr d)

        ppr (Call d)        = char 'C' <> parens (ppr d)


    instance Outputable Demands where
        ppr (Poly Abs) = empty
        ppr (Poly d)   = parens (ppr d <> char '*')
        ppr (Prod ds)  = parens (hcat (map ppr ds))

You do need to be able to read the pretty printing combinators. Here's a quick
cheat sheet; check http://hackage.haskell.org/packages/archive/pretty/1.0.1.0/doc/html/Text-PrettyPrint-HughesPJ.html
the basic idea.

    char ==> print a single character
    <>   ==> concatenate without adding a space
    parens x ==> put parentheses around x
    hcat ==> concatenate a list without adding a space

Cheers,
Edward

Excerpts from Johan Tibell's message of Wed Mar 07 18:41:42 -0500 2012:
> Edward, I have looked at that file before and it didn't make me much
> wiser, because I cannot map it to the output.
> 
> I find it's the parenthesis that confuses me the most. What does this mean?
> 
> C(U(LU(L)))
> 
> what about this?
> 
> U(SLLAA)LL
> 
> -- Johan