Variable types (à la System F) are not templates, though one can think of them as type schemata. A template
Testing is widespread in programming, but correctness in programming is mostly assured by construction. Testing prods a "black box"; this is appealing in that libraries are presented to users as black boxes, but, as we shall see, it is limited in what it can accomplish for deep logical reasons.
There are some ways that functional programming languages still lack when compared to C; in particular they fail to export their constructs for use in other languages.
Functional programmers vaunt the Curry-Howard(-Lambek) correspondence, as if it endorses the lambda calculus. In fact, it shows the limitations of computer science.
An established problem in functional programming is the question of evaluation
order (see Hudak, §2.2).
Haskell offers seq; which allows the programmer to magically introduce
dependencies in evaluation order and thence subvert lazy evaluation.
Sometimes this is necessary; see the foldl foldl' example.