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      6 
      7 Type-expander
      8 =============
      9 
     10 This project is written for
     11 [Typed/Racket](https://docs.racket-lang.org/ts-guide/) using Literate
     12 Programming.  See the “[Implementation of the type expander
     13 library](http://docs.racket-lang.org/type-expander/)” part of the [online
     14 documentation](http://docs.racket-lang.org/type-expander/) if you want to dig
     15 into the source.
     16 
     17 This library enhances typed/racket with type expanders, which are special
     18 macros that can appear where a type would normally be expected, and must
     19 expand to a type. Type expanders are to types what match expanders are to
     20 match patterns. It is based on Asumu Takikawa's [type
     21 expanders](https://github.com/racket/racket/compare/master...takikawa:tr-type-expander)
     22 (see also his [original pull request
     23 here](https://github.com/racket/racket/pull/604)).  Asumu Takikawa's work
     24 attempted to integrate type expanders directly into Typed/Racket.  This
     25 project instead implements type expanders as a library and works without any
     26 modification of the core Typed/Racket codebase. This shows the extensibility
     27 of Typed/Racket thanks to macros, and could serve as the basis for other
     28 projects which need to alter the manner in which Typed/Racket handles types.
     29 
     30 Installation
     31 ============
     32 
     33 ```
     34 raco pkg install --deps search-auto type-expander
     35 ```
     36 
     37 Usage example
     38 =============
     39 
     40 The `type-expander` is enabled by simply requiring the `type-expander` module
     41 in a `typed/racket` program.
     42 
     43     #lang typed/racket
     44     (require type-expander)
     45 
     46 For example, one can write the `(HomogeneousList n t)` type-expander, which
     47 expands to the type for a list of `n` elements of type `t`:
     48 
     49     (require (for-syntax syntax/parse racket/list))
     50     (define-type-expander (HomogeneousList stx)
     51       (syntax-parse stx
     52         [(_ t:expr n:nat)
     53          #`(List #,@(map (λ (x) #'t)
     54                          (range (syntax-e #'n))))]))
     55 
     56 It can then be used wherever a regular type is usually expected:
     57 
     58     (: five-strings (→ String (HomogeneousList String 5)))
     59     (define (five-strings x)
     60       (list x "a" "b" "c" "d"))
     61     
     62     (five-strings "hello")
     63     ;; => '("hello" "a" "b" "c" "d")
     64     
     65     (ann (five-strings "moon") (HomogeneousList String 5))
     66     ;; => '("moon"  "a" "b" "c" "d")