ucsd-progsys/liquidhaskell

Document that specs can depend on arguments of enclosing functions

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#2442 opened on Nov 19, 2024

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Description

Today I learned that the meaning of specs of local bindings can depend on arguments of an enclosing function. For instance, in the following function from the test suite, arg0 is available to the spec of bar.

{-@ foo :: x:_ -> y:_ -> {v:Int | v = x + y} @-}
foo :: Int -> Int -> Int
foo arg0 = bar
  where
    {-@ bar :: x:_ -> {v:Int | v = x + arg0} @-}
    bar arg1 = arg0 + arg1

It doesn't work for other things than arguments though, e.g. z is not in scope in the following spec.

{-@ foo :: x:_ -> y:_ -> {v:Int | v = x + y} @-}
foo :: Int -> Int -> Int
foo arg0 = bar
  where
    z = arg0
    {-@ bar :: x:_ -> {v:Int | v = x + z} @-}
    bar arg1 = arg0 + arg1

We should document the fact that arguments of enclosing functions are in scope. This is also true for other local bindings. See the comments below.

Contributor guide

Tech stack
None
Domain
documentation
Issue type
documentation
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
fresh
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
basic Haskellfamiliarity with Liquid Haskell
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
70
Research direction
The issue describes that arguments of enclosing functions are in scope for local bindings' specs, but other local bindings are not. To document this, examine the test suite (e.g., the example in the issue) and the comments below the issue for additional cases. The documentation should clarify which variables are available in the scope of a local spec, and mention that it does not work for non argument local bindings. Update the Liquid Haskell documentation accordingly, possibly in the section about refinement types or specifications.