unisonweb/unison

unhelpful error message when you forget to delay a function using an ability

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#641 opened on Aug 3, 2019

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 (4 comments) (0 reactions) (0 assignees)Haskell (304 forks)batch import
error-messagehelp wanted

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Description

Need a better error message for a common error case involving straightforward use of abilities.

Spot my mistake...

ability GetTime where
  systemTime : .base.io.EpochTime

test = GetTime.systemTime

Here's a clue:

  The expression in red needs the {GetTime} ability, but this location does not have access to any abilities.
  
      4 | test = GetTime.systemTime
  

.>

What do you mean that didn't help?! :-)

My definition should have been:

test _ = GetTime.systemTime

Note the underscore. (Then the type is correctly inferred to be '{GetTime} .base.io.EpochTime.) (Or the definition could be test = 'GetTime.systemTime.)

This is going to be a common error case: it's not very intuitive that test needs to take a unit argument just because it happens to be using an ability. Someone would hit this trying to write function that prints 'hello world' to screen.

(M1a)

Contributor guide

Research direction
Locate where the compiler generates the error message about missing ability access when a function is not delayed. Modify the error to suggest adding a unit argument or using the delayed syntax (').
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None
Domain
backend
Issue type
Bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
2
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1-3 hours
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Active
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Clear
Prerequisites
GitHaskell
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
75