Functional programming is based on the simplest of
models, namely that of finding the value of an
expression. This we meet in our first years at school,
when we learn to work out expressions like 8+(3-2) by
evaluating the two halves, 8 and (3-2), and adding the
results together. Functional programming consists of
our defining for ourselves functions like + which we
can then use to form expressions. 

We define these functions by means of equations, like 

addD a b = 2*(a+b)                                 (1)

which we use to calculate the value of an expression
like addD 2 (addD 3 4). We evaluate this using (1) to
replace occurrences of addD with their values, so that
we can write down a calculation of its value 

addD 2 (addD 3 4)
= 2*(2 + (addD 3 4))
= 2*(2 + 2*(3 + 4))
= 32

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