Add a better error message when decoding a bare None
#895
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Profpatsch
reviewed
Apr 20, 2019
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| instance Show CompileError where | ||
| show BareNone = | ||
| Data.Text.unpack $ | ||
| _ERROR <> ": A bare ❰None❱ is not valid \n\ |
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❰None❱ on its own is not valid
| \ \n\ | ||
| \Explanation: The conversion to JSON/YAML does not accept ❰None❱ in isolation as \n\ | ||
| \a valid way to represent ❰null❱. In Dhall, ❰None❱ is a function whose input is \n\ | ||
| \a type and whose output is an absent value of that type. \n\ |
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The “output is an absent value of that type” formulation is a bit confusing.
whose output is an `Optional` of that type
| \For example: \n\ | ||
| \ \n\ | ||
| \ \n\ | ||
| \ ┌─────────────────────────────────┐ This is a function whose result is an \n\ |
| \ \n\ | ||
| \ \n\ | ||
| \ ┌─────────────────────────────────┐ \n\ | ||
| \ │ None Natural : Optional Natural │ This is a valid ❰Optional❱ value (an \n\ |
| \ └─────────────────────────────────┘ absent ❰Natural❱ number in this case) \n\ | ||
| \ \n\ | ||
| \ \n\ | ||
| \The conversion to JSON/YAML only translates the latter form to ❰null❱. " |
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only translates the fully applied form (e.g. `None Natural`)
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I have a few suggestions that make the error message a bit clearer. |
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@Profpatsch: Since this is already merged, could you submit a pull request? |
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Fixes #896