On 4/11/07, Stefan O'Rear <[EMAIL PROTECTED]> wrote:
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).
"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).
(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.
> 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 :)
It would be
far better to do it yourself. Note that the method I am about to show
is exactly the same as that used by the standard showsPrec:
-- let +, - have infixl 1
-- let *, / have infixl 2
-- let ^ have infixr 3
-- let uminus have (nofix) 4
pprExpr :: Int -- ^ Precedence context - if you're like me no
-- explanation of this will make more sense than the
-- code
-> Expr -> Doc
pprExpr cx (Val str) = text str
pprExpr cx (a :+: b) = cparen (cx >= 1) $ pprExpr 1 a <+> char '+' <+> pprExpr
1 b
pprExpr cx (a :-: b) = cparen (cx >= 1) $ pprExpr 1 a <+> char '+' <+> pprExpr
1 b
pprExpr cx (a :*: b) = cparen (cx >= 2) $ pprExpr 2 a <+> char '+' <+> pprExpr
2 b
pprExpr cx (a :/: b) = cparen (cx >= 2) $ pprExpr 2 a <+> char '+' <+> pprExpr
2 b
pprExpr cx (a :^: b) = cparen (cx >= 3) $ pprExpr 3 a <+> char '+' <+> pprExpr
3 b
pprExpr cx (Negate a) = cparen (cx >= 4) $ char '-' <+> pprExpr 4 a
-- this is provided for ShowS under the name showsParen, but
-- unfortunately does not exist for Doc standardly
cparen :: Bool -> Doc -> Doc
cparen False = id
cparen True = parens
> showsPrec helps to take advantage of the precedence information.
> However, I don't find a way to remove parenthesis according to
> associativity.
A simple modification of the above code will do it:
pprExpr cx (Val str) = text str
pprExpr cx (a :+: b) = cparen (cx >= 1) $ pprExpr 0 a <+> char '+' <+> pprExpr
1 b
pprExpr cx (a :-: b) = cparen (cx >= 1) $ pprExpr 0 a <+> char '-' <+> pprExpr
1 b
pprExpr cx (a :*: b) = cparen (cx >= 2) $ pprExpr 1 a <+> char '*' <+> pprExpr
2 b
pprExpr cx (a :/: b) = cparen (cx >= 2) $ pprExpr 1 a <+> char '/' <+> pprExpr
2 b
pprExpr cx (a :^: b) = cparen (cx >= 3) $ pprExpr 3 a <+> char '^' <+> pprExpr
2 b
pprExpr cx (Negate a) = cparen (cx >= 4) $ char '+' <+> pprExpr 4 a
Handling line breaks is left as an excercise for the reader.
> I'm sure this kind of prettyprinting has been already done zillions of
> times in Haskell. Any suggestions?
Stefan
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