Let

an instruction included in QBASIC for unknown reasons, it had no function at all. You could use it to assign values to variables, but you didn't have to.

Example:

    LET a = 5

Would assign the value 5 to the variable a.
But you could do this:

    a = 5

With exactly the same result.. The use? You tell me.

(Mathematics:)

Beginning of a phrase binding a name to a certain value. The scope of the binding is understood (from clues from natural language, usually).

For instance:

Let x satisfy

x²=x+1.

Then 1/x=x-1.

(Computer science:)

(Lisp, functional programming languages:)
A special form for binding one or more values to names. The scope of the binding is the extent of the form (in older Lisps, e.g. Emacs Lisp, this is the dynamic extent, and the binding is dynamic binding; in functional programming languages and the newer Lisps, e.g. Common Lisp or Scheme, this is the static extent, and the binding is static binding).

Even in an impure functional language (Scheme, Lisp with static binding, ML and the like), this is not assignment in any form! All that happens is that certain values are given names for a short scope. (However, these names may later be assigned, e.g. with Scheme's set! special form; that is assignment...).

The Lispish form of let is:

(let ((VAR1 VAL1)
      ; ...
      (VALk VALk))
  BODY)

which evaluates the form(s) BODY with VAL_i named VAR_i. Note that all bindings occur simultaneously here.

This Lispish let is really just syntactic sugar for Scheme's

((lambda (VAR1 ... VARk)
    BODY)
 VAL1 ... VALk)

(The Lisp equivalent is essentially the same, but requires a funcall).

Other functional language's lets are the same conceptually, but generally with a more verbose concrete syntax.

(BASIC:)
Presumably inspired by the much earlier usage in Lisp (above), and exhibiting the usual confusion exemplified by BASIC's "it's too hard for beginners, let's tell them something wrong" attitude, BASIC uses the LET keyword as a prefix to assignment!

The original BASIC required the LET in

100 LET X=something

(it simplifies the parser somewhat). But LET quickly became optional (it doesn't make any real difference to the hardness of parsing BASIC).

In BASIC's defence, we could claim it's trying to echo the mathematical usage (at the top of this writeup). But that still embodies the same confusion between binding and assignment. This helps confuse anyone who grew up on BASIC (author of present writeup included) when they first hear of the Lispish let, or when they try to grok the C++ difference between assignment (operator=) and binding (the copy constructor).

Let (?), v. t. [OE.letten, AS. lettan to delay, to hinder, fr. laet slow; akin to D. letten to hinder, G. verletzen to hurt, Icel. letja to hold back, Goth. latjan. See Late.]

To retard; to hinder; to impede; to oppose.

[Archaic]

He was so strong that no man might him let. Chaucer.

He who now letteth will let, until he be taken out of the way. 2. Thess. ii. 7.

Mine ancient wound is hardly whole, And lets me from the saddle. Tennyson.

 

© Webster 1913.

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Let, n.

1.

A retarding; hindrance; obstacle; impediment; delay; -- common in the phrase without let or hindrance, but elsewhere archaic.

Keats.

Consider whether your doings be to the let of your salvation or not. Latimer.

2. Lawn Tennis

A stroke in which a ball touches the top of the net in passing over.

 

© Webster 1913.

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Let, v. t. [imp. & p. p. Let (Letted (?), [Obs].); p. pr. & vb. n. Letting.] [OE. leten, laeten (past tense lat, let, p. p. laten, leten, lete), AS. l&aemac;tan (past tense l&emac;t, p. p. l&aemac;ten); akin to OFries. l&emac;ta, OS. latan, D. laten, G. lessen, OHG. lazzan, Icel. lata, Sw. l�x86;ta, Dan. lade, Goth. l&emac;tan, and L. lassus weary. The original meaning seems to have been, to let loose, let go, let drop. Cf. Alas, Late, Lassitude, Let to hinder.]

1.

To leave; to relinquish; to abandon.

[Obs. or Archaic, except when followed by alone or be.]

He . . . prayed him his voyage for to let Chaucer.

Yet neither spins nor cards, ne cares nor frets, But to her mother Nature all her care she lets. Spenser.

Let me alone in choosing of my wife. Chaucer.

2.

To consider; to think; to esteem.

[Obs.]

Chaucer.

3.

To cause; to make; -- used with the infinitive in the active form but in the passive sense; as, let make, i. e., cause to be made; let bring, i. e., cause to be brought.

[Obs.]

This irous, cursed wretch Let this knight's son anon before him fetch. Chaucer.

He . . . thus let do slay hem all three. Chaucer.

Anon he let two coffers make. Gower.

4.

To permit; to allow; to suffer; -- either affirmatively, by positive act, or negatively, by neglecting to restrain or prevent.

In this sense, when followed by an infinitive, the latter is commonly without the sign to; as to let us walk, i. e., to permit or suffer us to walk. Sometimes there is entire omission of the verb; as, to let [to be or to go] loose.

Pharaoh said, I will let you go Ex. viii. 28.

If your name be Horatio, as I am let to know it is. Shak.

5.

To allow to be used or occupied for a compensation; to lease; to rent; to hire out; -- often with out; as, to let a farm; to let a house; to let out horses.

6.

To give, grant, or assign, as a work, privilege, or contract; -- often with out; as, to let the building of a bridge; to let out the lathing and the plastering.

The active form of the infinitive of let, as of many other English verbs, is often used in a passive sense; as, a house to let (i. e., for letting, or to be let). This form of expression conforms to the use of the Anglo-Saxon gerund with to (dative infinitive) which was commonly so employed. See Gerund, 2. " Your elegant house in Harley Street is to let." Thackeray. In the imperative mood, before the first person plural, let has a hortative force. " Rise up, let us go." Mark xiv. 42. " Let us seek out some desolate shade." Shak.

To let alone, to leave; to withdraw from; to refrain from interfering with. -- To let blood, to cause blood to flow; to bleed. -- To let down. (a) To lower. (b) To soften in tempering; as to let down tools, cutlery, and the like.<-- to let (someone) down. to disappoint (someone) by filing to perform as expected. --> -- To let drive or fly, to discharge with violence, as a blow, an arrow, or stone. See under Drive, and Fly. -- To let in or into. (a) To permit or suffer to enter; to admit. (b) To insert, or imbed, as a piece of wood, in a recess formed in a surface for the purpose. To let loose, to remove restraint from; to permit to wander at large. -- To let off (a) To discharge; to let fly, as an arrow; to fire the charge of, as a gun. (b) To release, as from an engagement or obligation. [Colloq.] To let out. (a) To allow to go forth; as, to let out a prisoner. (b) To extend or loosen, as the folds of a garment; to enlarge; to suffer to run out, as a cord. (c) To lease; to give out for performance by contract, as a job. (d) To divulge. -- To let slide, to let go; to cease to care for. [Colloq.] " Let the world slide." Shak.

 

© Webster 1913.

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Let, v. i.

1.

To forbear.

[Obs.]

Bacon.

2.

To be let or leased; as, the farm lets for $500 a year. See note under Left, v. i.

To let on, to tell; to tattle; to divulge something. [Low] -- To let up, to become less severe; to diminish; to cease; as, when the storm lets up. [Colloq.]

 

© Webster 1913.