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Diffstat (limited to 'txr.1')
| -rw-r--r-- | txr.1 | 83 |
1 files changed, 83 insertions, 0 deletions
@@ -28659,6 +28659,89 @@ applies inside a form. Of course, lexical operator macros do not shadow symbol macros under any circumstances. +.coNP Operator @ placelet +.synb +.mets (placelet >> ({( sym << place )}*) << body-form *) +.syne +.desc +The +.code placelet +macro binds lexically scoped symbol macros in such +a way that they behave as aliases for places +denoted by place forms. + +Each +.meta place +must be an expression denoting a syntactic place. The +corresponding +.meta sym +is established as an alias for the storage location which that place denotes, +over the scope of the +.metn body-form -s. + +This binding takes place in such a way that each +.meta place +is evaluated exactly once, only in order to determine its +storage location. The corresponding +.meta sym +then serves as an alias for that location, over the +scope of the +.metn body-form -s. +This means that whenever +.meta sym +is evaluated, it stands for the value of the storage +location, and whenever a value is apprently stored into +.metn sym , +it is actually the storage location which receives it. + +Note: certain kinds of places, notably +.cblk +.meti (force << promise ) +.cble +expressions, must be accessed before they can be stored, +and this restriction continues to hold when those +places are accessed through +.code placelet +aliases. + +Note: +.code placelet +differs from +.code symacrolet +in that the forms themselves are not aliased, but the storage +locations which they denote. +.code (symacrolet ((x y) z) +performs the syntactic substitution of symbol +.code x +by form +.codn y , +whereever +.code x +appears inside +.code z +as an evaluated form, and is not shadowed by any inner binding. +Whereas +.code (placelet ((x y)) z) +generates code which arranges for +.code y +to be evaluated to a storage location, and syntactically replaces occurrences +of +.code x +with a form which directly denotes that storage location, +wherever +.code x +appears inside +.code z +as an evaluated form, and is not shadowed by any inner binding. +Also, +.code x +is not necessarily substituted by a single, fixed form, +as in the case of +.codn symacrolet . +Rather it may be substituted by one kind of form when it +is treated as a pure value, and another kind of form +when it is treated as a place. + .coNP Operator @ tree-bind .synb .mets (tree-bind < macro-style-params < expr << form *) |