The document discusses challenges in producing useful error messages for liquid type systems like LiquidHaskell, Aeon, LiquidJava, and MLVP. It suggests improving error messages by only showing relevant context, substituting useless variables, simplifying redundant assertions, evidencing the minimal failing assertion, interactively expanding context, and indicating assertion provenance. Inspiration is drawn from the Lean 4 IDE and work by Tesla Zhang on simplifying proof context displays. The overall goal is to help developers better understand errors by filtering relevant information and simulating logical implications, rather than requiring them to do the work of an SMT solver.

1. Understandable and
Useful Error Messages
for Liquid Types
Alcides Fonseca, Catarina Gamboa, João David,
Guilherme Espada, Paulo Canelas
Funded by FCT projects:
UIDB/00408/2020, UIDP/00408/2020
LISBOA-01-0247-FEDER-045915 and
EXPL/CCI-COM/1306/2021

2. GOAL
Complaining about how hard it is to produce useful
error messages for:
• LiquidHaskell
• Aeon
• LiquidJava
• MLVP
Suggestions to improve error messages
2
80%
20%

3. LIQUID TYPES
• Base Types: int, bool,
fl
oat, ...
• Re
fi
nement Types: {x:Int | x > 3}
• Dependent Types: (x:Int) -> {y:Int | y > x}
3
https://ucsd-progsys.github.io/liquidhaskell-blog/

4. CHALLENGES
4
• Everything in the context may be relevant
• Developers need to select what may
be relevant
• There is a lot of redundancy (e.g., len v >
0 && len v >= 0)
• Developers need to
fi
lter relevant
assertions
• To understand errors, developers need to
simulate the logical implications from the
context.
• Developers are doing the work of the
SMT solver!

5. CHALLENGES
5
• Liquid Type Checking relies heavily
on rewrites:
• Internal rewrite to ANF
• Re
fi
nements renamed in
function calls

6. AEON
6
def minimizeGoal(m:Int) -> r:Bool | m > -10 {
fi
b(m) > 30
}
def f(i:Int, j:Int) -> k:Int | k > I+j and minimizeGoal(k) {
??
}
Alcides Fonseca, Paulo Santos, and Sara Silva. "The Usability Argument for Refinement Typed Genetic Programming." International Conference on Parallel Problem
Solving from Nature. Springer, Cham, 2020.
SMT-based Program Synthesis
Fitness Function for
Property-Based Testing
?? :: {k:Int | k > I+j && k > -10}
Another indirection layer!

7. LIQUIDJAVA
7
@Re
fi
nement("x > 10")
int x = 11;
x = x - 1;
Catarina Gamboa, Paulo Canelas, Christopher Timperley, Alcides Fonseca. "User-driven design and evaluation of Liquid Types in Java." HATRA @ SPLASH 2021
Another indirection layer in the context!
Context:
x_global | x_global > 10
x_local_1 | x_local_1 == 11
x_local_2 | x_local_2 == x_local - 1
x == 11 - 1 → x > 10

8. MLVP
8
Many indirection layers!

9. IDEAS FOR IMPROVEMENT
1. Show only relevant context
2. Substitute useless variables
3. Simplify redundant assertions
4. Evidence the minimal failing assertion
5. Allow to interactively expand the context
6. Evidence assertion provenance (colors?)
9

10. INSPIRATION
10
Lean 4 VSCode Extension

11. INSPIRATION
11
Tesla Zhang (https://twitter.com/OwoTizusa/status/1424159921172410372)

12. INSPIRATION
12
Tesla Zhang (https://twitter.com/OwoTizusa/status/1396787442015817730)

13. Understandable and
Useful Error Messages
for Liquid Types
Alcides Fonseca👨💻 , Catarina Gamboa👩🔬, João David,
Guilherme Espada👨🔬, Paulo Canelas👨💻