-ddump-simpl prints type applications with trailing whitespace (#17643) · Issues · Glasgow Haskell Compiler / GHC

-ddump-simpl prints type applications with trailing whitespace

If you compile the following code with -ddump-simpl enabled:

{-# LANGUAGE TypeApplications #-}

true :: Bool
true = id @Bool True

You'll get something like this:

-- RHS size: {terms: 2, types: 1, coercions: 0, joins: 0/0}
true :: Bool
true = id @ Bool True

There's one key difference between the original code and the dumped Core: the latter puts an extra space between the @ and Bool, whereas the former does not. This used to be just a minor curiosity, but now that GHC Proposal 229 is implemented, id @ Bool True is no longer code that will even parse correctly.

Granted, Core is not exactly the same as source Haskell, but the @ syntax in Core has enough similarities to source Haskell's TypeApplications that it seems reasonable that we should try to make the two notations coincide. In light of this, I propose that we change -ddump-simpl's pretty-printer to display true like this:

-- RHS size: {terms: 2, types: 1, coercions: 0, joins: 0/0}
true :: Bool
true = id @Bool True

That is, do not put any space between the @ and Bool, just as TypeApplications now requires. Does this sound like a reasonable suggestion?

If you compile the following code with `-ddump-simpl` enabled:

```hs
{-# LANGUAGE TypeApplications #-}

true :: Bool
true = id @Bool True
```

You'll get something like this:

```hs
-- RHS size: {terms: 2, types: 1, coercions: 0, joins: 0/0}
true :: Bool
true = id @ Bool True
```

There's one key difference between the original code and the dumped Core: the latter puts an extra space between the `@` and `Bool`, whereas the former does not. This used to be just a minor curiosity, but now that [GHC Proposal 229](https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0229-whitespace-bang-patterns.rst) is implemented, `id @ Bool True` is no longer code that will even parse correctly.

Granted, Core is not exactly the same as source Haskell, but the `@` syntax in Core has enough similarities to source Haskell's `TypeApplications` that it seems reasonable that we should try to make the two notations coincide. In light of this, I propose that we change `-ddump-simpl`'s pretty-printer to display `true` like this:

```hs
-- RHS size: {terms: 2, types: 1, coercions: 0, joins: 0/0}
true :: Bool
true = id @Bool True
```

That is, do not put any space between the `@` and `Bool`, just as `TypeApplications` now requires. Does this sound like a reasonable suggestion?