Haskell Src Exts Parsing Multiple Statements

Question

I would like to parse a list of Haskell statements. For instance, suppose I have the following code:

let a = b
    c = e
out <- return 3

I'd like a function, for instance parseStmts, which can return this in some parsed format.

I've looked into haskell-src-exts and saw parseStmt. This works for a single statement. It has type parseStmt :: String -> ParseResult Stmt, and if you try parseStmt "let a = 3", the result is a successful ParseOk. However, if you provide multiple statements, this function complains because there is more than one statement in the string.

How do I parse multiple statements, without wrapping them in a do block? Alternatively, how can I find the places in a string which are separations of Haskell statements, so I can separate them and then use parseStmt from haskell-src-exts?

Thanks!

bheklilr · Accepted Answer · 2013-10-11 20:00:57Z

2

You're looking for parseExp, although the output is a bit large:

> :m + Language.Haskell.Exts.Parser
> parseExp "do\n  let a = b\n      c = e\n  out <- return 3\n  return $ a + c + out"
ParseOk (Do [LetStmt (BDecls [PatBind (SrcLoc {srcFilename = "<unknown>.hs", srcLine = 2, srcColumn = 7}) (PVar (Ident "a")) Nothing (UnGuardedRhs (Var (UnQual (Ident "b")))) (BDecls []),PatBind (SrcLoc {srcFilename = "<unknown>.hs", srcLine = 3, srcColumn = 7}) (PVar (Ident "c")) Nothing (UnGuardedRhs (Var (UnQual (Ident "e")))) (BDecls [])]),Generator (SrcLoc {srcFilename = "<unknown>.hs", srcLine = 4, srcColumn = 3}) (PVar (Ident "out")) (App (Var (UnQual (Ident "return"))) (Lit (Int 3))),Qualifier (InfixApp (Var (UnQual (Ident "return"))) (QVarOp (UnQual (Symbol "$"))) (InfixApp (InfixApp (Var (UnQual (Ident "a"))) (QVarOp (UnQual (Symbol "+"))) (Var (UnQual (Ident "c")))) (QVarOp (UnQual (Symbol "+"))) (Var (UnQual (Ident "out")))))])

I had to add the return $ a + c + out to the end or else it throws an error, since it wouldn't be considered a valid do block otherwise.

answered Oct 11, 2013 at 20:00

bheklilr

54.2k6 gold badges112 silver badges166 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

Andrew Gibiansky Over a year ago

I considered this as a solution, but it requires adding a do, adding a return at the end, and indenting everything for the do. It's doable (get it? do-able?), but kinda ugly, and was hoping for a nicer solution.

bheklilr Over a year ago

@AndrewGibiansky You are wanting to parse correct haskell source code, right? If you don't indent properly and add the return statement, then you can't construct an AST from it. Are you instead wanting to simply parse the code into a less formal structure than the AST?

bheklilr Over a year ago

@AndrewGibiansky I'm not too familiar with using this library, but it looks like you could use the lexTokenStream function from Language.Haskell.Exts.Lexer module, it'll turn the code into tokens, but it still may be unwieldy to work with. This looks like a job for lenses.

Andrew Gibiansky Over a year ago

I think I'm going to go ahead and use this hack. I need this to parse things that I'm feeding to GHCi. Given a block of text, I need to extract the statements, separate them, and then feed each one to GHCi to evaluate it - and to do this I need to be able to separate the statements. Thanks!

Daniel Wagner · Accepted Answer · 2013-10-11 21:41:12Z

0

I don't think haskell-src-exts offers a ready-made function doing what you want, so one way or another you're going to have to write some of your own parsing code. That said, not all is lost. You may have to hack on haskell-src-exts itself to expose a few of its internals, but it should not be unduly difficult to throw together -- a few hours of work to get something decent if you're already familiar with whatever parsing technology it uses (alex/happy, I think?), or double it if you have to learn the parsing technology, too.

I'm sure some patches to the package to make this kind of thing easier would be welcomed with open arms, as well.

answered Oct 11, 2013 at 21:41

Daniel Wagner

156k10 gold badges231 silver badges392 bronze badges

Collectives™ on Stack Overflow

Haskell Src Exts Parsing Multiple Statements

2 Answers 2

4 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

4 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related