Re: [Haskell-cafe] ghci's choice of names for type variables

Joachim is right that, when you use a function that's already defined somewhere, GHC will use its choice of type variable names. That explains all the 'a's you observe. As for the others, `t` is GHC's letter of choice most of the time, and it's hard to predict (even for me, with pretty deep knowledge of the type checker) when it will use `p`. Bottom line: don't think about this too much -- there's not much of a signal in the noise. :) Richard

On Oct 30, 2021, at 7:06 AM, Julian Bradfield wrote:

I have tried to answer this by google and the list archives, but without success, though it wouldn't surprise me if there is a post buried somewhere.

This is GHCI 8.10.7.

When ghci infers types, it sometimes produces types with "a", with "t", and with "p" (and maybe others), as in the following set of examples:

Prelude> h (x : xs) = x Prelude> :t h h :: [a] -> a Prelude> foo f x = not(f x) Prelude> :t foo foo :: (t -> Bool) -> t -> Bool Prelude> bar f x = (f x) + 1 Prelude> :t bar bar :: Num a => (t -> a) -> t -> a Prelude> barb f x g y = (f x)+(g y) Prelude> :t barb barb :: Num a => (t1 -> a) -> t1 -> (t2 -> a) -> t2 -> a Prelude> gar f x = f x Prelude> :t gar gar :: (t1 -> t2) -> t1 -> t2 Prelude> fooa x = x Prelude> :t fooa fooa :: p -> p

What is its rationale? I have attempted to find it in the typechecker code, and I see things that suggest "t" is something to do with tau types (monotypes?), and "p" has something to do with levels, but going from basic Haskell and a modest theoretical acquaintance with System F to being able to read the GHCi type-checker is several steps too far!

Can somebody give me a brief explanation of what's going on? In particular, is there actual information about the types in the choice of letters, or is it just incidental information about the way type inference proceeded?

Thanks, Julian.

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