ENH Early check for in-block assignments

ChangeLog

@@ -1,3 +1,6 @@
+Version 1.2.0+
+	* Add early check that block assignments are always to block variables
+
 Version 1.2.0 2020-07-12 by luispedro
 	* Add load_fastq_directory to builtin functions
 	* Better messages when using `lock1`

NGLess/Validation.hs

@@ -1,4 +1,4 @@
-{- Copyright 2013-2018 NGLess Authors
+{- Copyright 2013-2020 NGLess Authors
  - License: MIT
  -}
 {-# LANGUAGE FlexibleContexts #-}
@@ -51,6 +51,7 @@ validate mods expr = case errors of
             ,validateNGLessVersionUses
             ,validatePureFunctions
             ,validateWriteOName
+            ,validateBlockAssignments
             ]
 
 {- Each checking function has the type
@@ -341,3 +342,19 @@ validateNGLessVersionUses mods sc = case nglVersion <$> nglHeader sc of
             guard $ not (T.null rest)
             (minV, _) <- rightToMaybe $ T.decimal (T.tail rest)
             return (majV, minV)
+
+
+-- Check that only block variables are assigned inside a block
+validateBlockAssignments :: [Module] -> Script -> Writer [T.Text] ()
+validateBlockAssignments _ (Script _ es) = forM_ es validateBlockAssignments1
+validateBlockAssignments1 :: (Int, Expression) -> Writer [T.Text] ()
+validateBlockAssignments1 (lno, e) = case e of
+    Assignment _ e' -> validateBlockAssignments1 (lno, e')
+    FunctionCall (FuncName fname)  _ _ (Just block) -> let [var] = blockVariable block
+                                        in recursiveAnalyse (checkAssignmentOnlyTo fname lno var) (blockBody block)
+    _ -> return ()
+checkAssignmentOnlyTo fname lno v@(Variable n) e = case e of
+    Assignment v' _
+        | v /= v' -> tell1lno lno ["Inside blocks, only the block variable (in this case `", n, "`) can be assigned to",
+                                    " (when analysing function `", fname, "`)."]
+    _ -> return ()

tests/error-block-assignment/reassign-in-block.ngl

@@ -0,0 +1,9 @@
+ngless '1.2'
+input = fastq('sample.fq')
+trim = 3
+input = preprocess(input) using |read|:
+   read = read[trim:]
+   trim = 5
+   if len(read) < 10:
+       discard
+write(input, ofile='output.fq')

tests/error-block-assignment/sample.fq

@@ -0,0 +1,12 @@
+@IRIS:7:1:17:394#0/1
+GTCAGGACAAGAAAGACAANTCCAATTNACATT
++IRIS:7:1:17:394#0/1
+aaabaa`]baaaaa_aab]D^^`b`aYDW]aba
+@IRIS:7:1:17:800#0/1
+GGAAACACTACTTAGGCTTATAAGATCNGGTTGCGG
++IRIS:7:1:17:800#0/1
+ababbaaabaaaaa`]`ba`]`aaaaYD\\_a``XT
+@IRIS:7:1:17:1479#0/1
+TGAAAGAT
++IRIS:7:1:17:1479#0/1
+__a_X]``