Re: [Haskell-cafe] Prettyprinting infix expressions with HughesPJ

10 Apr 2007


      On Wed, Apr 11, 2007 at 01:53:49AM +0200, Alfonso Acosta wrote:
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On 4/11/07, Stefan O'Rear  wrote:
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Your use of 'have' is slightly suspicious here.  That said, the rest
of your problem looks very un-homework-y, so I'll look at it.
It's for my masters thesis (big piece of badly-specified homework  if
you want to think about it like that :)). I used "have" cause I'm
coding an embedded VHDL compiler and I really "have" to pretty print
VHDL in the backend :) (the more readable the code I get the better).
Hehe.  We're happy to solve subproblems.  It's the
copied-verbatim-from-the-book homework problems we mind.
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"Associativity" is ambiguous here.  Do you mean:
1 + 2 + 3   =>  (1 + 2) + 3      (Associativity of parsing)
This is what I mean. Say "-" is right associative in the target
language, that allows to remove the parenthesis of the following
expression "1 - (1 - 2)" and just write "1 - 1 - 2", but probably the
code would be less readable since a human doesn't normally take that
in account (I tend to forget those things and keep writing parenthesis
all the time anyway).
It's pretty easy to make this happen - just pretend (-) is
non-associative.
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(1 + 2) + 3 == 6 == 1 + (2 + 3)  (Associativity of functions)
I don't see how taking this in account would make the output more readable.
Suppose we receive Lit 1 :+: (Lit 2 :+: Lit 3).  Would you rather read
1 + 2 + 3 or 1 + (2 + 3)? 

NB: Beware of floating points!
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data Expr = Val String |
                 -- Binary operators (using infix constructors)
                 Expr :+: Expr  | Expr  :-: Expr  |
                 Expr :*: Expr  | Expr  :/: Expr  |
                 Expr :^: Expr |
                 -- Unary operators
                 Negate Expr
I'm using HughesPJ for the rest of my AST (not just expressions) but
the library doesn't provide any mechanism to help coding this kind of
prettyprinter so I decided to simply use the standard showsPrec and
then feed HughesPJ with the obtained text.
That seems very counter-productive.  By using showsPrec, you lose all
the information that could be used to guide line breaks.
Yes, you're totally right. Thanks a lot for your code :)
Stefan