feat: hand-written fault-tolerant parser, LSP completion and signature help

CONTRIBUTING.md

@@ -3,4 +3,6 @@
 You can update snaphshots with the
 `UPDATE_SNAPS=true` variable while running the tests
 
-If you need to update the grammar, you can generate the parser by running `just generate` (you'll need to install the antlr4 command first)
+The Numscript parser (`internal/parser`) is hand-written, so that it can recover from syntax errors: the LSP needs a meaningful AST for programs that are being typed. The [Numscript.g4](Numscript.g4) and [Lexer.g4](Lexer.g4) grammars are still the reference for the syntax, and they generate the previous, ANTLR-based parser (`internal/parser/internal/legacy`). The tests check that both parsers agree on every program of the repository, so if you change the syntax, update both the hand-written parser and the grammar.
+
+You can generate the ANTLR parsers by running `just generate` (you'll need to install the antlr4 command first)

Justfile

@@ -13,8 +13,8 @@ tidy:
     @go mod tidy
 
 generate:
-    @antlr4 -Dlanguage=Go Lexer.g4 Numscript.g4 -o internal/parser/antlrParser -package antlrParser
-    @mv internal/parser/antlrParser/_lexer.go internal/parser/antlrParser/lexer.go
+    @antlr4 -Dlanguage=Go Lexer.g4 Numscript.g4 -o internal/parser/internal/legacy/antlrParser -package antlrParser
+    @mv internal/parser/internal/legacy/antlrParser/_lexer.go internal/parser/internal/legacy/antlrParser/lexer.go
     @antlr4 -Dlanguage=Go IR.g4 -o internal/ir/internal/syntax/antlrParser -package antlrParser
 
 tests:

codecov.yml

@@ -1,2 +1,3 @@
 ignore:
-  - "internal/parser/antlrParser"
+  - "internal/parser/internal/legacy"
+  - "internal/ir/internal/syntax/antlrParser"

go.mod

@@ -21,7 +21,7 @@ require (
 	github.com/google/uuid v1.6.0 // indirect
 	github.com/inconshreveable/mousetrap v1.1.0 // indirect
 	github.com/invopop/jsonschema v0.13.0 // indirect
-	github.com/kr/pretty v0.3.1 // indirect
+	github.com/kr/pretty v0.3.1
 	github.com/kr/text v0.2.0 // indirect
 	github.com/mailru/easyjson v0.9.2 // indirect
 	github.com/maruel/natural v1.3.0 // indirect

internal/analysis/check.go

@@ -178,6 +178,10 @@ func (r *CheckResult) GetVarDeclType(decl parser.VarDeclaration) Type {
 }
 
 func (r *CheckResult) unifyNodeWith(expr parser.ValueExpr, t Type) {
+	if expr == nil {
+		// a missing node of an incomplete program
+		return
+	}
 	exprT := r.GetExprType(expr)
 	r.unify(expr.GetRange(), exprT, t)
 }