Add a filter for simple fractions

Makefile

@@ -1,11 +1,29 @@
 SOURCES = $(wildcard *.md)
 OUT = $(SOURCES:.md=.pdf)
 
-all: $(OUT)
+HS_FILTERS_NAMES = nice-frac
+PY_FILTERS_NAMES = pandoc-svg.py
+EXT_FILTERS = pandoc-crossref
+
+HS_FILTERS = $(addprefix filters/, $(HS_FILTERS_NAMES))
+PY_FILTERS = $(addprefix filters/, $(PY_FILTERS_NAMES))
+FILTERS = $(PY_FILTERS) $(HS_FILTERS) $(EXT_FILTERS)
+
+.SUFFIXES: .hs
+
+all: $(HS_FILTERS) $(OUT)
 
 $(OUT): $(SOURCES)
-	pandoc --smart -t latex $< --filter=filters/pandoc-svg.py --filter=pandoc-crossref -o $@
+	pandoc --smart -t latex $< $(addprefix --filter=, $(FILTERS)) -o $@
+
+$(HS_FILTERS): $(addsuffix .hs, $(HS_FILTERS))
+	ghc --make -O2 $< -o $@	
 
 clean:
 	rm -f $(OUT)
+	rm -f $(addprefix filters/, $(HS_FILTERS))
 	find . -name "*.pandoc.pdf" -delete
+
+deepclean: clean
+	rm -f $(HS_FILTERS)
+	rm -f filters/*.hi filters/*.o

README.md

@@ -12,6 +12,8 @@ Includes various Pandoc filters:
 * Include [pandoc-crossref](https://github.com/lierdakil/pandoc-crossref) for
   easy numbering and referencing.
 
+* Convert fractions like "(n/k)" to "\frac{n}{k}" when n and k are integers in
+  math environment.
 
 ## TODO
 
@@ -19,9 +21,5 @@ Includes various Pandoc filters:
 LaTeX equivalent (\cos and so on) when in a mathematical environment,
 then taking out the useless leading `\`.
 
-* not having to use \frac{}{} to write fractions, but instead
-automatically convert simple fractions in math environment to \frac{}{}
-(for instance convert "(1 / 2)" to "\frac{1}{2}")
-
 * anything else like this that could help writing more readable LaTeX
 code :)

filters/nice-frac.hs

@@ -0,0 +1,26 @@
+-- nice-frac.hs
+
+import Text.Pandoc.JSON
+import Text.Regex
+
+-- Usefull regex elements
+space :: String
+space = "[ \\t\\n\\r]*"
+num :: String
+num = "([0-9]+)"
+
+-- Substitute latex fractions like '\frac{42}{1}' for (42 / 1)
+fracReg :: Regex
+fracReg = mkRegex $
+  "\\(" ++ space ++ num ++ space ++ "/" ++ space ++ num ++ "\\)"
+frac :: String -> String
+frac s = subRegex fracReg s "\\frac{\\1}{\\2}"
+
+-- Apply the substitution to all Latex parts on the AST
+niceFrac :: Inline -> Inline
+niceFrac (Math t s) = Math t (frac s)
+niceFrac x = x
+
+main :: IO ()
+main = toJSONFilter niceFrac
+