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Head Disk Assembly

<hardware, storage> (HDA) A sealed, high capacity mainframe hard disk with integral heads, as opposed to a removable disk.

(1999-01-13)

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header

1. The portion of a packet, preceding the actual data, containing source and destination addresses, error checking and other fields.

2. The part of an electronic mail message or news article that precedes the body of a message and contains, among other things, the sender's name and e-mail address and the date and time the message was sent.

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Head Normal Form

<theory, reduction> (HNF) A term describing a lambda expression whose top level is either a variable, a data value, a built-in function applied to too few arguments, or a lambda abstraction whose body is not reducible. I.e. the top level is neither a redex nor a lambda abstraction with a reducible body.

An expression in HNF may contain redexes in argument postions whereas a normal form may not.

Compare Weak Head Normal Form.

(2003-01-08)

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head normalisation theorem

Under the typed lambda-calculus, beta/delta reduction of the left-most redex (normal order reduction) is guaranteed to terminate with a head normal form if one exists. See also Church-Rosser theorem.

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heads down

[Sun] Concentrating, usually so heavily and for so long that everything outside the focus area is missed. See also hack mode and larval stage, although this mode is hardly confined to fledgling hackers.

[Jargon File]

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head-strict

<theory> A head-strict function will not necessarily evaluate every cons cell of its (list) argument, but whenever it does evaluate a cons cell it will also evaluate the element in the head of that cell. An example of a head-strict function is

	beforeZero :: [Int] -> [Int]
	beforeZero []     = []
	beforeZero (0:xs) = []
	beforeZero (x:xs) = x : beforeZero xs

which returns a list up to the first zero.

This pattern of evaluation is important because it is common in functions which operate on a list of inputs.