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Building Language Processors Using Pure Functional Programming Techniques

This guide explores the construction of language processors utilizing pure functional programming principles. It details the implementation of recognizers for terminal words, alternatives, sequences, and complex sequences. With practical examples such as handling tokens and interpreting complex expressions, the text demonstrates how to build a solid backend for language processing. Functions such as `term`, `num`, and operators like `$orelse` and `$then` are examined to facilitate understanding of parsing strategies, offering insights for developers interested in functional programming applications in language recognition.

jasmine-lancaster
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Building Language Processors Using Pure Functional Programming Techniques

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  1. How to build language processors in a pure functional programming language

  2. Recognizers – for terminals term wd (tok:toks) = [toks], if tok = wd = [], otherwise term x [] = [] e.g. term “one” [“one”,“two”,“three”] => [[“two”,”three”]] term “two” [“one”,“two”,“three”] => [] Now one = term “one” two = term “two” plus = term “+” etc

  3. Recognizers – alternatives (p $orelse q) toks = p toks ++ q toks e.g. one $orelse two [“one”,”four”] => [[“four”]] one $orelse two [“two”,”six”] => [[“six”]] one $orelse two [“three”,”six”] => [] num = one $orelse two $orelse three …

  4. Recognizers – sequence (p $then q) toks = concat (map p (q toks)) e.g. (one $then two) [“one”,”two”,”three”] => [[“three”]] (one $then two) [“one”,”six”,”four”] => [] num $then num [“one”,”two”,”three”] => [[“three”]]

  5. Recognizers – complex add_seq = num $orelse (num $then plus $then add_seq) e.g. add_seq [“one”,”+”,”three”,”+”,“six”] => [[“+”,”three”,“+”,”six”], [“+”,”six”], [] ] add_seq [“plus”,”six”,”four”] => []

  6. Interpreters – for terminals term (wd,val) (tok:toks) = [(val,toks)], if tok = wd = [], otherwise term x [] = [] e.g. term (“one”,1) [“one”,“two”,“three”] => [(1,[“two”,”three”]]) Now one = term (“one”,1) two = term (“two”,2) plus = term (“+”,(+)) etc

  7. Interpreters – alternatives (p $orelse q) toks = p toks ++ q toks e.g. one $orelse two [“one”,”four”] => [(1,[“four”])] one $orelse two [“two”,”six”] => [(2,[“six”])] one $orelse two [“three”,”six”] => [] num = one $orelse two $orelse three …

  8. Interpreters – sequence (p1 $then p2) toks = [((v1,v2),t2) | (v1,t1) <- p1 toks; (v2,t2) <- p2 t1] e.g. (one $then two) [“one”,”two”,”three”] => [((1,2),[“three”])] (one $then plus $then two) [“one”,”plus”,”two”.”three”] => [(1,((+),2)),[“three”])]

  9. Interpreters – complex add_seq = num $orelse ((num $then plus $then add_seq) $apply_rule app_op) (p $applyrule f) inp = [ (f v, r) | (v, r) <- p inp ] app_op (v1, (op, v2)) = op v1 v e.g add_seq [“one”,”+”,”three”,”+”,“six”] => [(1,[“+”,”three”,“+”,”six”]), (4,[“+”,”six”]), (10,[]) ]

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