I'm asking this more out of curiosity. What does the inferred type look like when a type variable name includes an integer? For example:

foo :: forall r1 r2 r3. r1 -> r2 -> r3 -> r1
foo r _ _ = r

It'd end up suffixing another digit regardless.

forall r25 r36. r25 -> r36 -> Int

I'm thinking maybe this could be helped by adding an ':

forall r2'5 r3'6. r2'5 -> r3'6 -> Int

Good idea, but I think ' is used more frequently than integers when quickly distinguishing one type var from another:

foo :: forall sym a r r'.
  ( Row.Cons sym a r r'
  , IsSymbol sym
  ) => ....

which would produce

forall sym4 a5 r'6 r''7. ...

So, there's a few conflicting issues here:

• No matter which separater one uses, this issue will always arise if one terminates the ty var name with that same separator.
• If one accounts for the ty var name conditionally (e.g. "if it ends with integer, insert '; if ends with ', insert _; ..."), the inferred ty var names will no longer be consistent (e.g. a1, a'1, a_1)
• There are only so many chars one can use before appending extra chars to the ty var name makes it longer than it needs to be and thus unreadable / noisy (e.g. a_tyVarNum1, a'_tyVarNum2, a__tyVarNum3).
• While prepending the integer to the front of the ty var would make it easily distinguishable, a ty var name cannot start with an integer, so it breaks copy-paste workflows.

So, I think this issue I've brought up should be ignored. This PR does address the larger problem, namely, that forall t0 t1 t2 t3 is harder to read and compare to its original function than forall a0 b0 c0 d0.

Also, I was curious to see what happens when multiple functions using the same ty var names are used together:

-- @shouldWarnWith MissingTypeDeclaration
module Main where

foo :: forall a b c d. a -> b -> c -> d -> a
foo a _ _ _ = a

bar :: forall a b c d. a -> b -> c -> d -> a
bar a _ _ _ = a

baz a x y = bar (foo a 2 3 4) (foo a 2 3 4) (foo x y a a)

produces forall c14 d15 b25 d27. d27 -> c14 -> b25 -> d15 -> d27

Warning found:
in module �[33mMain�[0m
at tests/purs/warning/4256v2.purs:10:1 - 10:58 (line 10, column 1 - line 10, column 58)

  No type declaration was provided for the top-level declaration of �[33mbaz�[0m.
  It is good practice to provide type declarations as a form of documentation.
  The inferred type of �[33mbaz�[0m was:
  �[33m                                                         �[0m
  �[33m  forall c14 d15 b25 d27. d27 -> c14 -> b25 -> d15 -> d27�[0m
  �[33m                                                         �[0m

in value declaration �[33mbaz�[0m

See https://github.com/purescript/documentation/blob/master/errors/MissingTypeDeclaration.md for more information,
or to contribute content related to this warning.

@rhendric Any other feedback on this? I believe the issues you raised have been resolved.