A compiler takes text and returns a meaning. The text is the input and the meaning is the output, and the whole subject is the shape of the thing in between.
About 1 min of reading
Write the three parts
Write the input
Before you run it, say out loud what it will print. Then open the answer.
Write the middle
Before you run it, say out loud what it will print. Then open the answer.
Write the answer
Before you run it, say out loud what it will print. Then open the answer.
A compiler with no middle is a function that cannot be tested in pieces. The middle is what makes the pieces testable.
A compiler you can read at every step. Reading at every step is the reason the compiler is built rather than used.
Text has no shape. A number in text is a number only after something decides it is one.
The compiler either says the text is a program or names where it is not. Both answers are decisions, and a decision is what a test can check.
The middle is a value with a shape. A shape that is written down is a shape that can be printed and argued with.