Discussion: Static Safety via Distinct Interfaces for HsSyn ASTs

[Simon Peyton Jones]

I’m keen NOT to introduce these layers of indirection.  I think they make the code harder to understand.

Can you give an example function or two, and what it would look like under the different approaches.

(1)-(3) appears to be three different approaches, but I don’t think that’s what you intend.  I think there are only two: add the indirection layer or not?

S

From: Shayan Najd [mailto:sh.najd at gmail.com]
Sent: 23 August 2017 13:26
To: ghc-devs at haskell.org
Cc: Simon Peyton Jones <simonpj at microsoft.com>; Alan & Kim Zimmerman <alan.zimm at gmail.com>
Subject: Discussion: Static Safety via Distinct Interfaces for HsSyn ASTs

In this thread, I am going to raise a topic for discussion. Please share your opinions and related experiences.

Evaluation of type families within HsSyn ASTs, such as `PostTc`, with a fixed phase index, such as `GhcPs`, gives us distinct ASTs at the *compile-time*.
However, when programming with these ASTs, we use patterns, such as `HsMultiIf :: PostTc p Type -> [LGRHS p (LHsExpr p)] -> HsExpr p` that are shared among phases.
We can
(1) introduce a layer of abstraction providing a set of type and pattern synonyms specific to each phase, such as `PsMultiIf :: [LPsGRHS  LPsExpr] -> PsExpr`; and
(2) updating code working on ASTs of specific phase to use the interface specific to the phase, such as by changing prefixes from `Hs` to `Ps` and by removing unused variables and placeholders; and
(3) leaving untouched code working uniformly on ASTs of different phases (i.e., the generic functions in Trees that Grow terminology), such as the existing functions whose types are polymorphic on phase index.

Some comments:

- It can be done gradually and smoothly: we add three separate files in HsSyn (per each phase) containing the phase-specific interfaces, and gradually import them and do the changes per module.
- Using the interfaces is optional: code using the current method (e.g., using `HsMultiIf`) should work just fine.
- It introduces a layer of indirection and three more files to maintain.
- It makes code working on HsSyn ASTs, such as the renamer, appear cleaner as placeholders and similar machinery are abstracted away by the interfaces (e.g., no need to import bits and pieces of `HsExtension`)
- In theory, there should be zero impact on GHC's runtime performance.

I am myself undecided about its benefit-cost ratio, but willing to at least implement the phase-specific interfaces.
For me, abstracting away all the `PostRn` stuff, `Out` prefixed constructors, and dummy placeholders from the front-end code is the most valuable.

Yours,
  Shayan







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