bad pretty-printing of types with promoted data types

> :set -XDataKinds
> :set -XPolyKinds
> data Proxy k = Proxy
> _ :: Proxy '[ 'True ]
error:
  Found hole: _ :: Proxy '['True]
>             _ :: Proxy '['True]
error:
    Invalid type signature: _ :: ...
    Should be of form <variable> :: <type>

Alternatively, this could be attributed to the parser/lexer doing an insufficient job there.

Trac metadata

Trac field	Value
Version	8.2.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
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Blocking
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changed weight to 5

added Tbug Trac import labels

Are you sure you put that in right? With GHC 8.2.1 I get

Prelude> _ :: Proxy '[ 'True ]

<interactive>:3:1: error:
    • Found hole: _ :: Proxy '['True]
    • In the expression: _ :: Proxy '[ 'True]
      In an equation for ‘it’: it = _ :: Proxy '[ 'True]
    • Relevant bindings include
        it :: Proxy '['True] (bound at <interactive>:3:1)

But if I miss out the space it parses '[' as a character literal.

Prelude> _ :: Proxy '['True ]

<interactive>:4:1: error:
    Invalid type signature: _ :: ...
    Should be of form <variable> :: <type>

Hard to avoid that.

Make sense? I'm inclined to say 'wont-fix'.

I am fine with [ being lexed as it is, so i don't mind a wont-fix for that aspect. But why does the error message for syntactically valid input contain source fragments that are not syntactically valid?

Just as a test for how input is transformed (on 8.2.1 as well):

Prelude> _ :: Proxy '[  'True ]                           -- 2 spaces in input

<interactive>:8:1: error:
    • Found hole: _ :: Proxy '['True]                     -- 0 spaces
    • In the expression: _ :: Proxy '[ 'True]             -- 1 spaces
      In an equation for ‘it’: it = _ :: Proxy '[ 'True]  -- 1 spaces
    • Relevant bindings include
        it :: Proxy '['True] (bound at <interactive>:8:1) -- 0 spaces

The zeros are problematic, and i think the ones only avoid the problem because they don't really re-layout, they just merge whitespace (at least that's what i suspect; haven't looked at the source).

This is just a deficiency of GHC's pretty-printer. It should be relatively straightforward to change it such that GHC inserts a space between '[ and 'True. See the pprIfaceTyList function, which is responsible for pretty-printing promoted lists.

Note that similar considerations also apply for promoted tuple constructors:

λ> f :: Proxy _ -> Proxy '( 'True, 'True); f x = x

<interactive>:5:12: error:
    • Found type wildcard ‘_’
        standing for ‘'('True, 'True) :: (Bool, Bool)’
      To use the inferred type, enable PartialTypeSignatures
    • In the type signature: f :: Proxy _ -> Proxy '( 'True,  'True)

So an analogous fix would be needed for the pprTuple function.

added newcomer label

assigned to @trac-andreash

I started working on this in https://phabricator.haskell.org/D4746.

Trac metadata

Trac field	Value
Differential revisions	- → D4746

mentioned in commit 767536cc

closed

changed milestone to %8.6.1

Trac metadata

Trac field	Value
Resolution	Unresolved → ResolvedFixed

mentioned in commit ae2c9b40

added Pnormal label

mentioned in issue #22488 (closed)