"we write expressions in infix form" - why not "we write expressions in the infix form"?

Question

From Real World Haskell:

We can immediately start entering expressions, to see what ghci will do with them. Basic arithmetic works similarly to languages like C and Python: we write expressions in infix form, where an operator appears between its operands.

Why is there no the before "infix"? This is quite a definite form of writing expressions. Is it because it is given a definition after a comma (" where an operator appears between its operands")?

Or is it because form with the will change its meaning to "a printed or typed document with blank spaces for insertion of required or requested information"?

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P.S. A related question: 'in gas state' vs. 'in the gas state'.

(P.P.S. stashing a memorable usage example for myself: "Alf in pog form")

Answer 1

Because "infix form" is a non-count noun phrase. That is, it's not a phrase that would make sense in the plurality, such as referring to "the infix forms". There is only one infix form, and it is what you will be writing the expression in.

Compare the sentence using "exponential notation" or "hexadecimal format". Neither of these would typically make sense used as plurals because there is just one being referred to ("We write numbers in the exponential notations." & "We write numbers in hexadecimal formats." are both bad.)

I think it also sounds stranger because 'in' is used consecutively. But saying, "we write expressions in the infix form" is as incorrect as saying, "we write expressions in the exponential notation."