[ghostwriter]

Haskell, because:

1. every other mainstream language is one step below the Abstractions Ladder and Haskell knowledge converts naturally to any other $mainstream flavour of the day.

2. every other academic/research language either directly or indirectly assumes you are familiar with higher-kinded interfaces and compositions that all practitioners of Haskell are familiar with, which converts naturally to the desire to achieve more (dependent types and formal proofs, effect systems, and so on).

3. map/fold/traverse are ubiquitous patterns across all FP languages, even their syntax and signatures hardly ever vary.

4. you begin to understand and appreciate simple mathematical concepts in the context of industrial programming.