Input variables and Scheme - GNU LilyPond
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#### 11.1.1 Input variables and Scheme

The input format supports the notion of variables: in the following example, a music expression is assigned to a variable with the name `traLaLa`.

    traLaLa = { c'4 d'4 }

There is also a form of scoping: in the following example, the `\layout` block also contains a `traLaLa` variable, which is independent of the outer `\traLaLa`.

    traLaLa = { c'4 d'4 }
    \layout { traLaLa = 1.0 }

In effect, each input file is a scope, and all `\header`, `\midi`, and `\layout` blocks are scopes nested inside that toplevel scope.

Both variables and scoping are implemented in the GUILE module system. An anonymous Scheme module is attached to each scope. An assignment of the form

    traLaLa = { c'4 d'4 }

is internally converted to a Scheme definition

    (define traLaLa Scheme value of “... ”)

This means that input variables and Scheme variables may be freely mixed. In the following example, a music fragment is stored in the variable `traLaLa`, and duplicated using Scheme. The result is imported in a `\score` block by means of a second variable `twice`:

    traLaLa = { c'4 d'4 }

    #(define newLa (map ly:music-deep-copy
      (list traLaLa traLaLa)))
    #(define twice
      (make-sequential-music newLa))

    { \twice }

In the above example, music expressions can be `exported' from the input to the Scheme interpreter. The opposite is also possible. By wrapping a Scheme value in the function `ly:export`, a Scheme value is interpreted as if it were entered in LilyPond syntax. Instead of defining `\twice`, the example above could also have been written as

    ...
    { #(ly:export (make-sequential-music (list newLa))) }

Scheme code is evaluated as soon as the parser encounters it. To define some scheme code in a macro (to be called later), use

    #(define (nopc)
      (ly:set-option 'point-and-click #f))

    ...
    #(nopc)
    { c'4 }

##### Bugs

Mixing Scheme and LilyPond identifiers is not possible with the `--safe` option.

This page is for LilyPond-2.8.8 (stable-branch).

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