No way at all to set fixity for infix operators defined in template haskell

[guest]

There appears to be no way to create fixity declarations in template haskell.

As a result, I can define a complicated sequence of infix identifiers in a template Haskell expression, but I still have to go through and hand-enter the {infixl, infixr, infix} declarations at the top. Admittedly the fixities it creates won't be used by the current module, but importers of the module should still see them.

There is a precedent for template Haskell handling some fixity concerns because all of the Info pieces of template Haskell tell you the fixities of the infix operators in question.

[guest]

There is no way at all to give a fixity to an infix operation defined in template Haskell.

They aren't in scope in the module itself at the time infix* declarations are parsed.

They aren't considered in scope and thus available for changing fixity in other modules.

[ekmett@…]

minimalist example

[simonpj]

This seems like a reasonable feature request. Would anyone like to tackle it?

Simon

[reinerp]

I propose to implement this by adding the following constructor to Language.Haskell.TH.Dec:

 data Dec = 
     ...
     | FixityD Name Fixity
     ...

and making the appropriate changes in Convert.lhs and DsMeta.hs.

This definition allows constructing fixity declarations which refer to operators defined in other modules (say). It seems easiest for Template Haskell not to be concerned by this, and defer checking for such errors to the renamer.

Thoughts?

[simonpj]

That looks plausible to me. The question of fixity declarations for things defined elsewhere is no different than for pragmas or type signatures, so I don't think it's an issue.

Simon

[goldfire]

I'm writing a TH program that, among other things, takes operators defined by a client program and makes new operators based on those. Having the ability to produce a fixity declaration would be great.

[mgsloan1]

++ I want this too!

My suggestion would be to make it the following:

 data Dec = 
     ...
     | FixityD Fixity Name
     ...

As nearly all of the TH constructors use the order of the syntax for the order of the fields.

[simonpj@…]

commit ef2491a378bbe0fcaea7aa2ad69bc82a838f6a59

Author: Simon Peyton Jones <simonpj@microsoft.com>
Date:   Wed Mar 14 17:28:32 2012 +0000

    Add fixity declarations to Template Haskell (Trac #1541)
    
    There is an accompanying patch to the template-haskell library

 compiler/deSugar/DsMeta.hs |   75 ++++++++++++++++++++++++++++++++++++-------
 compiler/hsSyn/Convert.lhs |   30 ++++++++++++-----
 2 files changed, 83 insertions(+), 22 deletions(-)

[simonpj]

I've finally done this, closing a 5-year-old ticket. Hurrah.

Simon

[simonpj@…]

commit 55372bcd814aa17413ec1fd8ca23a0b5064e784c

Author: Simon Peyton Jones <simonpj@microsoft.com>
Date:   Wed Mar 14 17:28:32 2012 +0000

    Add fixity declarations to Template Haskell (Trac #1541)
    
    There is an accompanying patch to the template-haskell library

 compiler/deSugar/DsMeta.hs |   75 ++++++++++++++++++++++++++++++++++++-------
 compiler/hsSyn/Convert.lhs |   30 ++++++++++++-----
 2 files changed, 83 insertions(+), 22 deletions(-)