An expression that GHC can type but cannot assign to a name

#1

Long story short, see the gist. The exercise is to make GHC accept the function deasils.

The expression I refer to in the title is deasilsB . deasilsA.

GHC can type it:

% ghci-9.2.2 Squeeze.hs
GHCi, version 9.2.2: https://www.haskell.org/ghc/  :? for help
Loaded GHCi configuration from /home/kindaro/code/dotfiles/ghci.conf
[1 of 1] Compiling Main             ( Squeeze.hs, interpreted )
Ok, one module loaded.
λ :type deasilsB . deasilsA
deasilsB . deasilsA
  :: …

GHC cannot assign it to a name:

λ f = deasilsB . deasilsA

<interactive>:2:1: error: …

Evidence that the computation is correct:

λ deasilsB (deasilsA (['a', 'b'], True) :: Squeezed '[[ ]] (Char, Bool))
[('a',True),('b',True)]

What is going on? How can I have my deasils?

This is a continuation of my previous work on iterated currying and mapping.

#2

If you enable AllowAmbiguousTypes and NoMonormorphismRestriction, and if you leave out the type signature of deasils, then the source file actually type checks. But adding the inferred type as a signature gives a type error about ambiguity.

That is really odd. I would report this to the GHC issue tracker.

#3

See issue to GHC.