Csci 318 Module 23

Front 1

thunk

Back 1

As convenient terminology/jargon, when we use a zeroargument function to delay evaluation we call the function a thunk.

Front 2

call-by-need

Back 2

Suppose we have a large computation that we know how to perform but we do not know if we need to perform it. Other parts of the program know where the result of the computation is needed and there may be 0, 1, or more different places. If we thunk, then we may repeat the large computation many times. But if we do not thunk, then we will perform the large computation even if we do not need to. To get the “best of both worlds,” we can use a programming idiom known by a few different (and perhaps technically slightly different) names: lazy-evaluation, call-by-need, promises.

Front 3

call-by-value

Back 3

Some languages, most notably Haskell, use this approach for all function calls, i.e., the semantics for function calls is different in these languages: If an argument is never used it is never evaluated, else it is evaluated only once. This is called call-by-need whereas all the languages we will use are call-by-value (arguments are fully evaluated before the call is made).

Front 4

stream

Back 4

A stream is an infinite sequence of values.

Front 5

memoization

Back 5

An idiom related to lazy evaluation that does not actually use thunks is memoization.

Front 6

macro definition

Back 6

A macro definition introduces some new syntax into the language. It describes how to transform the new syntax into different syntax in the language itself.

Front 7

macro expansion

Back 7

The semantics of a macro use is to replace the macro use with the appropriate syntax as defined by the macro definition. This process is often called macro expansion because it is common but not required that the syntactic transformation produces a larger amount of code.