Stop pretending studying arrow theory improves your apping

Given two functors S,T:C→B, a natural transformation τ:S→T is a function which assigns to each object c of C an arrow τ_c=τc:Sc→TC of B in such a way that every arrow f:c→c' in C yields a diagram
ﾠﾠﾠﾠﾠﾠﾠﾠﾠﾠﾠτc
c ﾠﾠ Sc---→Tc
|ﾠﾠﾠﾠﾠﾠﾠﾠ|ﾠﾠﾠﾠﾠﾠﾠ|
|fﾠﾠﾠﾠﾠSf↓ﾠﾠﾠﾠﾠ↓Tf
↓ﾠﾠﾠﾠﾠﾠﾠ|ﾠﾠτc'ﾠﾠ|
c', ﾠﾠﾠ Sc'--→Tc'

which is commutative. When this holds, we also say that τ_c=τc:Sc→Tc is natural in c. If we think of the functor S as giving a picture in B of (all the objects and arrows of) C, then a natural transformation τ is the set of arrows mapping (or, translating) the picture S to the picture T, with all squares (and parallelograms!) like that above commutative:
ﾠaﾠﾠﾠﾠﾠﾠﾠﾠﾠﾠSa----------→Ta
ﾠ|ﾠ╲fﾠﾠﾠﾠﾠﾠﾠ ﾠ|ﾠ╲Sfﾠﾠﾠﾠﾠﾠﾠﾠﾠ|ﾠ╲Tf
ﾠ|ﾠﾠﾠ↘ﾠﾠﾠﾠﾠﾠﾠ|ﾠﾠﾠ↘ﾠﾠﾠτbﾠﾠﾠ|ﾠﾠﾠ↘
ﾠ|ﾠﾠﾠﾠﾠbﾠﾠﾠﾠ ﾠ|ﾠﾠﾠﾠSb----------→Tb
ﾠ|ﾠﾠﾠ╱ﾠﾠﾠﾠﾠﾠﾠ|ﾠﾠﾠ╱ﾠﾠﾠﾠﾠﾠﾠﾠﾠ|ﾠﾠﾠ╱
↓ﾠ↙ﾠﾠﾠﾠﾠﾠ ﾠ↓↙Sgﾠﾠﾠﾠﾠﾠﾠﾠ↓ﾠ↙Tg
ﾠcﾠﾠﾠﾠﾠﾠﾠﾠﾠﾠSc----------→Tc

Fuck I love Haskell. Really.

You will join an expert team in London, and demonstrable experience in typed FP (Haskell, OCaml, F# etc) is required. We have around 2.5 million lines of Haskell, and our own Haskell compiler. In this context we look for skill and taste in typed functional programming to capture and abstract over complex, messy systems.

To all those retards who say Haskell is not useful in production: take it up the ass.

This is a “front office” finance role – meaning you will work on the trading floor, directly with traders, building software to automate their work and improve their efficiency. The role is highly development focused, and you will use Haskell for almost all tasks: data analysis, market data publishing, database access, web services, desktop GUIs, large parallel tasks, quantitative models, solvers, everything. This is a fast paced role – code you write today will be deployed within hours to hundreds of users and has to work.