Parser: prettyIndex context lines

src/error.ts

@@ -21,10 +21,6 @@ export const prettySyntaxError = (error: SyntaxError): string => {
 export const prettySourceMessage = (message: string, index: number, source: Source): string => {
     const location = indexToLocation(index, source)
     const locationStr = `${location ? `${source.filename}:${prettyLocation(location)}` : '<unknwon location>'}`
-    const indent = ' '.repeat(4)
-    return `\
-${prettyIndex(index, source)}
-${message}
-${indent}at ${locationStr}
-`
+    const locationMsg = `${' '.repeat(2)}at ${locationStr}`
+    return [message, locationMsg, '\n' + prettyIndex(index, source, 1) + '\n'].join('\n')
 }

src/location.ts

@@ -1,5 +1,6 @@
 import { isNewline } from './lexer/lexer'
 import { Source } from './source'
+import { range } from './util/array'
 
 export interface LocationRange {
     start: number
@@ -28,21 +29,21 @@ export const indexToLocation = (index: number, source: Source): Location | undef
     return undefined
 }
 
-/**
- * TODO: context lines parameter
- */
-export const prettyIndex = (index: number, source: Source): string => {
+export const prettyIndex = (index: number, source: Source, context: number = 0): string => {
     const start = indexToLocation(index, source)
     if (!start) return '<outside of a file>'
+    const highlight = ' '.repeat(6 + start.column) + '^'
+    const linesBefore = range(0, context).map(i => start.line + i - context).map(i => prettyLine(i, source))
+    const linesAfter = range(0, context).map(i => start.line + i + context).map(i => prettyLine(i, source))
 
+    return [linesBefore.join('\n'), prettyLine(start.line, source), highlight, linesAfter.join('\n')].join('\n')
+}
+
+export const prettyLine = (lineIndex: number, source: Source): string => {
     const lines = source.str.split('\n')
-    const line = lines[start.line]
-    const highlight = ' '.repeat(start.column) + '^'
-    const lineNum = `${(start.line + 1).toString()} | `.padStart(6)
-    return `\
-    
-${lineNum}${line}
-${' '.repeat(lineNum.length)}${highlight}`
+    const line = lines[lineIndex]
+    const lineNum = `${(lineIndex + 1).toString()} | `.padStart(6)
+    return lineNum + line
 }
 
 export const prettyLocation = (location: Location): string => `${location.line + 1}:${location.column + 1}`

src/util/array.ts

@@ -0,0 +1 @@
+export const range = (from: number, to: number): number[] => new Array(to - from).fill(0).map((_, i) => i + from)