On 4/11/07, Stefan O'Rear
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